I have to wonder how much you would follow this logic elsewhere though. It says “establish prayer” about 40 times in the Quran. Why not pray 40 times? What else does the Quran say a number of times that you wouldn't do that many times? Why do you presume the five times is in one day and not week?
Why also ignore explicit verses that state the number of prayers and their times over “alluded to” numbers?
The overall point being, how hard would it be for Allah to state that there are 5 daily prayers in the Quran? But the Quran neglects that while telling Muslims to establish prayer scores of times. It doesn't seem to be a size of book or an easy to memorise issue.
The non-explicit manner here is obvious to me. It's to get us use to the way the Quran and Sunnah compliment each other. Quran obviously could've easily said five. However, with knowing it's five from the Sunnah, it becomes too much of a coincidence that where I showed you to not be seen as verifying the Sunnah.
If you are to rigid with Quran, it won't be lead you to many doors opening many doors.
The repetition here would be over-kill - as in we are told to face the Qibla twice in two verses, and one between them, in succession, it's not dispersed. For the repetition to not be over-kill and purposeful, makes sense that it's alluding to the five times. Without that, it seems like over-kill in repetition.
God could've easily said things explicitly in many things. He could have said "Ali is the chosen leader after Mohammad (s)" in Quran. There is many layers to why he didn't.
If he was explicit in details of Salah - then people would have more of an argument as to why the names of the Twelve Imams (a) should be there in the Quran.
As God had reasons not to be explicit about Ali (a), Hassan (a) and Hussain (a) but suffice in clear signs regarding them in Quran and the concept of the chosen family and authority in a way there is room for twisting and turning, it makes sense, that he forms the Quran in a way you need to the Sunnah for details of other essential things like Salah.
However, just like the Imams (a) can be verified in the Quran, and the Sunnah is more explicit, the same is true of Salah.
So you got to keep this in mind as well.
And reasons why God was not explicit with Imams (a) include the need to safeguard the Quran in a natural way. It's also the reasons why the Imams (a) didn't reveal scripture, as opposition would put question the integrity of the Quran as their scripture which would have to be on par with it would be put into question.
Im not suggesting that you cannot have commentary but that you shouldn't need it as an essential part of your faith. Whatever, it is that humans have written in Hadith that is essential to the faith could easily have been said in a far better way by Allah and included in the Quran.
If not every single thing, then at least, the basics.
The most essential part of Quran is Welayat of Ali (a). See the above explanation as to why it's not explicit in that. However, most of the Quran is about Welayat Ali (a) albeit mostly indirectly.