Because people choose what they want to believe. Where many see proofs, others see differently.
The God guides whom He Wills. He says that you can’t make the blind see, the deaf to hear, & He says that even if He were to open a gate to the heavens & they were to ascend & descend, they will say my eyes are deceiving me, I have been bewitched.
And He has open up many things to scientist that many of us do not get to see, i.e. the inside of the human & animal bodies, how perfectly they are put together & function.
They study the 2 seas that meet but don’t mix, that was revealed over 1400yrs. ago in The Holy Quran. Muhammad didn’t have equipment to study & know those two waters, that from an eye’s view, look like one body of water, don’t & can’t mix, many have tried, even by putting both waters in a glass:
two seas meet but don’t mix
1. [He has let loose the two seas (the salt water & the sweet) meeting together].
2. [Between them is a barrier which none of them can transgress].
3. [Then which of the Blessings of your Lord will you both (jinns & men) deny?]
4. [Out of them both come out pearl & coral].
5. [Then which of the Blessings of your Lord will you both deny?]
2. (The Holy Quran-Surah Al-Rahman, Verse 19-3)
3. (Yusuf Ali Translation of The Holy Quran http://www.islam101.com/quran/yusufAli/QURAN/5.htm)
They look out into this universe, as for as The God Will allow. Seeing how it functions perfectly. Earth not crashing into the sun, etc.
Yet, with all that & more, that they are seeing with their own eyes, they still don’t believe.