God is unseen, and so he can force that the signs of the unseen/hidden and make it manifest, I agree. But why doesn't he do so?
Salam
The Quran contains an explanation to all things. One of these is how it emphasizes on the issue of proof and day of judgment, and why God to some degree hides his proofs. On the day of judgment, the signs of God's Authority there is no room for denial:
لَيْسَ لِوَقْعَتِهَا كَاذِبَةٌ
There is to it's passing no denier.
(56:2)
اسْتَجِيبُوا لِرَبِّكُمْ مِنْ قَبْلِ أَنْ يَأْتِيَ يَوْمٌ لَا مَرَدَّ لَهُ مِنَ اللَّهِ ۚ مَا لَكُمْ مِنْ مَلْجَإٍ يَوْمَئِذٍ وَمَا لَكُمْ مِنْ نَكِيرٍ | Respond to your Lord before there comes a day for which there will be no revoking from Allah. On that day you will have no refuge, nor will you able to have denial| Ash-Shura : 47
When the proof is manifest and is forced upon people, their will is no longer sincere, for this reason the Quran says:
إِنَّ السَّاعَةَ آتِيَةٌ أَكَادُ أُخْفِيهَا لِتُجْزَىٰ كُلُّ نَفْسٍ بِمَا تَسْعَىٰ | Indeed the Hour is bound to come: I will have it hidden, so that every soul may be rewarded by what for it strives. | Taa-Haa : 15
The proof is related to the power in the Prophets and Witnesses:
وَقِيلَ مَنْ ۜ رَاقٍ
And it will be said who is the enchanter?
إِلَىٰ رَبِّكَ يَوْمَئِذٍ الْمَسَاقُ
Towards your Lord is the driving
And said:
وَجَاءَتْ كُلُّ نَفْسٍ مَعَهَا سَائِقٌ وَشَهِيدٌ | Every soul will come accompanied by a driver and a witness: | Qaaf : 21
And said:
وَيَوْمَ نَبْعَثُ مِنْ كُلِّ أُمَّةٍ شَهِيدًا ثُمَّ لَا يُؤْذَنُ لِلَّذِينَ كَفَرُوا وَلَا هُمْ يُسْتَعْتَبُونَ | The day We shall raise a witness from every nation, the faithless will not be permitted nor will they be asked to propitiate | An-Nahl : 84
And said:
وَيَوْمَ نَبْعَثُ فِي كُلِّ أُمَّةٍ شَهِيدًا عَلَيْهِمْ مِنْ أَنْفُسِهِمْ ۖ وَجِئْنَا بِكَ شَهِيدًا عَلَىٰ هَٰؤُلَاءِ ۚ وَنَزَّلْنَا عَلَيْكَ الْكِتَابَ تِبْيَانًا لِكُلِّ شَيْءٍ وَهُدًى وَرَحْمَةً وَبُشْرَىٰ لِلْمُسْلِمِينَ | The day We raise in every nation a witness against them from among themselves, We shall bring you as a witness against these (people of your time). We have sent down the Book to you as a clarification of all things and as guidance, mercy and good news for the submitters | An-Nahl : 89
يَوْمَ نَدْعُو كُلَّ أُنَاسٍ بِإِمَامِهِمْ ۖ فَمَنْ أُوتِيَ كِتَابَهُ بِيَمِينِهِ فَأُولَٰئِكَ يَقْرَءُونَ كِتَابَهُمْ وَلَا يُظْلَمُونَ فَتِيلًا | The day We shall summon every group of people along with their Leader, then whoever is given his book in his right hand—they will read it, and they will not be wronged so much as a single date-thread. | Al-Israa : 71
And Mohammad (s) is been called the clear proof:
لَمْ يَكُنِ الَّذِينَ كَفَرُوا مِنْ أَهْلِ الْكِتَابِ وَالْمُشْرِكِينَ مُنْفَكِّينَ حَتَّىٰ تَأْتِيَهُمُ الْبَيِّنَةُ | The faithless from among the People of the Book and the polytheists were not set apart [from the community of the faithful] until the proof had come to them: | Al-Bayyina : 1
رَسُولٌ مِنَ اللَّهِ يَتْلُو صُحُفًا مُطَهَّرَةً | A Messenger from Allah reciting purified scriptures, | Al-Bayyina : 2
And the degree his proof is hidden and manifest, is up to God
وَيَقُولُونَ لَوْلَا أُنْزِلَ عَلَيْهِ آيَةٌ مِنْ رَبِّهِ ۖ فَقُلْ إِنَّمَا الْغَيْبُ لِلَّهِ فَانْتَظِرُوا إِنِّي مَعَكُمْ مِنَ الْمُنْتَظِرِينَ | They say, ‘Why has not some sign been sent down to him from his Lord?’ Say, ‘the hiding (of the sign) is for God. So wait. I too am waiting along with you.’ | Yunus : 20
And hence belief in the hidden is about belief in God's proof and signs:
الَّذِينَ يُؤْمِنُونَ بِالْغَيْبِ وَيُقِيمُونَ الصَّلَاةَ وَمِمَّا رَزَقْنَاهُمْ يُنْفِقُونَ | who believe in the hidden, maintain the prayer, and spend out of what We have provided for them; | Al-Baqara : 3
And said:
عَنِ النَّبَإِ الْعَظِيمِ | [Is it] about the Great Tiding, | An-Naba : 2
And regarding reward Mohammad (s) is accused of seeking:
وَلَتَعْلَمُنَّ نَبَأَهُ بَعْدَ حِينٍ | and you will surely learn its tidings in due time.’ | Saad : 88
Which is explained to be:
ذَٰلِكَ الَّذِي يُبَشِّرُ اللَّهُ عِبَادَهُ الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ ۗ قُلْ لَا أَسْأَلُكُمْ عَلَيْهِ أَجْرًا إِلَّا الْمَوَدَّةَ فِي الْقُرْبَىٰ ۗ وَمَنْ يَقْتَرِفْ حَسَنَةً نَزِدْ لَهُ فِيهَا حُسْنًا ۚ إِنَّ اللَّهَ غَفُورٌ شَكُورٌ | Such is the good news that Allah gives to His servants who have faith and do righteous deeds! Say, ‘I do not ask you any reward for it except the love of the kin.’ Whoever performs a good deed, We shall enhance its goodness for him. Indeed Allah is Forgiving, Appreciative. | Ash-Shura : 23
And hence this love is connected to goodness and faith in God as they are unseen signs of God that God manifests or hides to whatever degree he wishes.
And this is addresses why God hides and manifests proofs, but the question is why miracles in the past and not now?
The Quran says:
وَمَا مَنَعَنَا أَنْ نُرْسِلَ بِالْآيَاتِ إِلَّا أَنْ كَذَّبَ بِهَا الْأَوَّلُونَ ۚ وَآتَيْنَا ثَمُودَ النَّاقَةَ مُبْصِرَةً فَظَلَمُوا بِهَا ۚ وَمَا نُرْسِلُ بِالْآيَاتِ إِلَّا تَخْوِيفًا | Nothing keeps Us from sending signs except if the former people denied them. We gave Thamud the she-camel as an eye-opener, but they wronged her. We do not send the signs except as warning. | Al-Israa : 59
And hence previous generations actions have caused the situation in which the Guide who can perform such signs is hidden:
وَيَقُولُ الَّذِينَ كَفَرُوا لَوْلَا أُنْزِلَ عَلَيْهِ آيَةٌ مِنْ رَبِّهِ ۗ إِنَّمَا أَنْتَ مُنْذِرٌ ۖ وَلِكُلِّ قَوْمٍ هَادٍ | The faithless say, ‘Why has not some sign revealed regarding him from his Lord?’ You are only a warner, and there is a guide for every people. | Ar-Ra'd : 7
And God says:
اللَّهُ نُورُ السَّمَاوَاتِ وَالْأَرْضِ ۚ مَثَلُ نُورِهِ كَمِشْكَاةٍ فِيهَا مِصْبَاحٌ ۖ الْمِصْبَاحُ فِي زُجَاجَةٍ ۖ الزُّجَاجَةُ كَأَنَّهَا كَوْكَبٌ دُرِّيٌّ يُوقَدُ مِنْ شَجَرَةٍ مُبَارَكَةٍ زَيْتُونَةٍ لَا شَرْقِيَّةٍ وَلَا غَرْبِيَّةٍ يَكَادُ زَيْتُهَا يُضِيءُ وَلَوْ لَمْ تَمْسَسْهُ نَارٌ ۚ نُورٌ عَلَىٰ نُورٍ ۗ يَهْدِي اللَّهُ لِنُورِهِ مَنْ يَشَاءُ ۚ وَيَضْرِبُ اللَّهُ الْأَمْثَالَ لِلنَّاسِ ۗ وَاللَّهُ بِكُلِّ شَيْءٍ عَلِيمٌ | Allah is the Light of the heavens and the earth. The parable of His Light is a niche wherein is a lamp—the lamp is in a glass, the glass as it were a glittering star—lit from a blessed olive tree, neither eastern nor western, whose oil almost lights up, though fire should not touch it. Light upon light. Allah guides to His Light whomever He wishes. Allah draws parables for mankind, and Allah has knowledge of all things. | An-Noor : 35
فِي بُيُوتٍ أَذِنَ اللَّهُ أَنْ تُرْفَعَ وَيُذْكَرَ فِيهَا اسْمُهُ يُسَبِّحُ لَهُ فِيهَا بِالْغُدُوِّ وَالْآصَالِ | In houses Allah has allowed to be raised and wherein His Name is celebrated; He is glorified therein, morning and evening, | An-Noor : 36
رِجَالٌ لَا تُلْهِيهِمْ تِجَارَةٌ وَلَا بَيْعٌ عَنْ ذِكْرِ اللَّهِ وَإِقَامِ الصَّلَاةِ وَإِيتَاءِ الزَّكَاةِ ۙ يَخَافُونَ يَوْمًا تَتَقَلَّبُ فِيهِ الْقُلُوبُ وَالْأَبْصَارُ |Men whom neither trade nor bargaining distracts from the remembrance of Allah and the maintenance of prayer and the giving of zakat. They are fearful of a day wherein the hearts and the sights will be transformed, | An-Noor : 37
And said:
فَآمِنُوا بِاللَّهِ وَرَسُولِهِ وَالنُّورِ الَّذِي أَنْزَلْنَا ۚ وَاللَّهُ بِمَا تَعْمَلُونَ خَبِيرٌ | So have faith in Allah and His Apostle and the light, which We have revealed, and Allah is well aware of what you do. | At-Taghaabun : 8
Thus the philosophy of hiding and manifesting is explained in Quran. God's proof is always here, but to what degree he is hidden or manifest, is up to God. Right now there is no miracles in the open, but that's because of what previous generations did with them and oppressed the followers who believed in them.
I find that it's a major sign of Quran being from God, about how much it emphasized on the issue of proof, to what degree God manifests or hides it, and for what reasons. It shows God is aware people want proof.
Concerning the sky reality where is the light of God and spiritual lights, Quran also said:
وَإِنْ كَانَ كَبُرَ عَلَيْكَ إِعْرَاضُهُمْ فَإِنِ اسْتَطَعْتَ أَنْ تَبْتَغِيَ نَفَقًا فِي الْأَرْضِ أَوْ سُلَّمًا فِي السَّمَاءِ فَتَأْتِيَهُمْ بِآيَةٍ ۚ وَلَوْ شَاءَ اللَّهُ لَجَمَعَهُمْ عَلَى الْهُدَىٰ ۚ فَلَا تَكُونَنَّ مِنَ الْجَاهِلِينَ | And should their aversion be hard on you, find, if you can, a tunnel into the ground, or a ladder into sky, that you may bring them a sign. Had Allah wished, He would have brought them together on guidance. So do not be one of the ignorant. | Al-An'aam : 35
This shows if Mohammad (s) can connect people to this reality, he would, but it's not his fault people turn away.
Surah Hijr expands on why he doesn't connect them:
وَلَوْ فَتَحْنَا عَلَيْهِمْ بَابًا مِنَ السَّمَاءِ فَظَلُّوا فِيهِ يَعْرُجُونَ | Were We to open for them a gate of the sky, so that they could go on ascending through it, | Al-Hijr : 14
لَقَالُوا إِنَّمَا سُكِّرَتْ أَبْصَارُنَا بَلْ نَحْنُ قَوْمٌ مَسْحُورُونَ | they would surely say, ‘Indeed a spell has been cast on our eyes; indeed, we are a bewitched lot.’ | Al-Hijr : 15
وَلَقَدْ جَعَلْنَا فِي السَّمَاءِ بُرُوجًا وَزَيَّنَّاهَا لِلنَّاظِرِينَ | Certainly We have made great stars in the sky and adorned them for the onlookers, | Al-Hijr : 16
وَحَفِظْنَاهَا مِنْ كُلِّ شَيْطَانٍ رَجِيمٍ | and We have guarded it from every outcast Satan, | Al-Hijr : 17
إِلَّا مَنِ اسْتَرَقَ السَّمْعَ فَأَتْبَعَهُ شِهَابٌ مُبِينٌ | except someone who may eavesdrop, whereat there pursues him a manifest flame. | Al-Hijr : 18
And this returns to the issue of why proof is not forced upon people:
قَالَ يَا قَوْمِ أَرَأَيْتُمْ إِنْ كُنْتُ عَلَىٰ بَيِّنَةٍ مِنْ رَبِّي وَآتَانِي رَحْمَةً مِنْ عِنْدِهِ فَعُمِّيَتْ عَلَيْكُمْ أَنُلْزِمُكُمُوهَا وَأَنْتُمْ لَهَا كَارِهُونَ | He said, ‘O my people! Tell me, should I stand on a manifest proof from my Lord, and He has granted me His own mercy—though it be that you are blinded to it—shall we force it upon you while you are averse to it? | Hud : 28