There are often many factors, I'm sure scientific reasons being one of them. Each person has their own beliefs and reasons, and most are equally as passionate about their beliefs or non beliefs. I don't judge. I accept each person for their own reasons and beliefs. I'm certain you are opening up room for the answers with this question. lol
Some people have been through things that make them question if there's a God, what's the reason He'd put them through that. Some people need proof, and some people are super into science and don't see a way there is God.
They don't believe in him because they can't see him but ask yourself can you see air or do you feel it God is a feeling that's in with you and Yes I understand people are losing faith because of all the evil things that they are seeing if you don't believe what I am saying Look up Reverlation in the Bible it tells you everything that's happened to the world good or bad place a part in our world
They need absolute proof, even though they are drowning in it. They worry about the speck in our eyes, but they can't see past the log in their own. Seeing isn't believing. Can you see gravity? How do you know it isn't an invisible horse beast who keeps everything together? Can you see thoughts or feelings? How do we know these things exist? Because we experience them. Same with God. If I can't prove God to someone using physical evidence, then after some prayer, it is only a matter of time before God makes himself known to them.
The most common denominator is the undeniable absence of any evidence supporting a gods existence. Books, religious attempts to sell psuedoscience, and the flimsy concept of faith are not any sort of proof.
No, there could be many reasons. First, a person may have grown up with parents who didn't believe in God. They never went to Sunday school or church, never read the Bible, and never taught their children to do these things. Therefore, the child-like the parents-doesn't believe in God.
Second, a person may have gone to college and was taught that evolution is true and that God isn't needed since "nature" can account for everything. When this happens, belief in evolution takes the place of belief in God. (cont in comments)
( 55 ) And nothing has prevented the people from believing when guidance came to them and from asking forgiveness of their Lord except that there [must] befall them the [accustomed] precedent of the former peoples or that the punishment should come [directly] before them.
( 56 ) And We send not the messengers except as bringers of good tidings and warners. And those who disbelieve dispute by [using] falsehood to [attempt to] invalidate thereby the truth and have taken My verses, and that of which they are warned, in ridicule.
( 57 ) And who is more unjust than one who is reminded of the verses of his Lord but turns away from them and forgets what his hands have put forth? Indeed, We have placed over their hearts coverings, lest they understand it, and in their ears deafness. And if you invite them to guidance - they will never be guided, then - ever.
( 58 ) And your Lord is the Forgiving, full of mercy. If He were to impose blame upon them for what they earned, He would have hastened for them the punishment. Rather, for them is an appointment from which they will never find an escape. Quran (18:55-58)