How do you know that they are not?
Response: Let's say that I present to you a basketball and say that the ball can not bounce. You see the basketball and say that it can bounce. So I begin to provide evidence from historians and scholars to prove my stance and likewise you do the same.
So what is the end result? Naturally I will say that your facts are wrong and you can easily say the same to me. So how do we resolve this? Well, what better way to know if the basketball will bounce than to bounce it yourself?!! What better logic of
proof can be better than that?!!
In other words, test the logic. And this simple concept of testing logic is mentioned in the qur'an. In ch: 4:82 of the qur'an we read, "Will they not,then, meditate upon the Qur'an? Had it been from anyone other than Allah, they would surely have found therein much discrepancy". Also in ch.2:23 we read "And if you are in doubt as to what We have sent down to Our servant, then produce a chapter like it, and call upon your helpers beside Allah, if you are truthful".
Here we have two tests to prove the authenticity and truth of the qur'an. Once applied, you will come to learn that the qur'an is in fact from Allah and has never nor will it ever be corrupted. That established, we turn to ch.24:55 which says "Allah has promised to those among you who believe and do good works that He will, surely, make them Successors in the earth, as He made successors (of) those who were before them; and that He will, surely, establish for them their religion which He has chosen for them..." And in ch.17:81 we read "And proclaim 'Truth has come and falsehood has vanished. Verily, falsehood is bound to vanish".
So the qur'an establishes the fact that the false religions will fail in comparison to the truth. So the fact that the collection of Sahih Bukhari and Muslim is established as authentic by the muslim world over other hadiths and biographies shows that these other collections are indeed false.