Because there are many different religions in the world and not all of them believe in the same things. Then there are people who need to have solid evidence of something, in order to believe it, and while each of us might experience the spiritual in our own ways - in general there is not absolute verifiable proof of any specific deity (or the divine), so they choose not to believe lacking that solid evidence (which is perfectly fine as well).
the same reason children stop believing in santa, or the tooth fairy, they grow up and realies that things aren't always what they seem, they start notice god was nowhere to be seen when a fatal five car pile up happens, some lose faith, some never had it to start with, some think logically, some read the bible and think he is an evil person who doesn't deserve to be worshiped, some just don't see any reason for it, there are plenty or reasons why some people don't believe, in fact someone could say "why do some people believe in god" but the reasons I don't, 1) I was raised to believe in what ever I want to believe in, and god is not one of them 2) praying doesn't cure cancer so why do it 3) praying doesn't help people effected by a storm, if god is such a good person then why did he let the storm kill all those people in the first place 4) I'm not a puppet, And I don't expect to be treated as one 5) I'm not going to waste my time, It's as simple as that
2nd Corinthians 4:4 among whom the god of this system of things ( Satan ) has blinded the minds of the unbelievers that the illimination of the Glorious good news about the Christ who is the image of God might not shine through. compare 1 John 5:19
Jesus exposed the true motives of those who reject God when he said, "God's light came into the world, but people loved the darkness more than the light, for their actions were evil. All who do evil hate the light and refuse to go near it for fear their sins will be exposed" (John 3:19-20).
"The unbeliever has preferred to be by himself, without God, defying God, having God against him, and he shall have his preference. Nobody stands under the wrath of God save those who have chosen to do so. The essence of God's action in wrath is to give men what they choose, in all its implications: nothing more, and equally nothing less." J.I. Packer
Because to have integrity is to apply the same level of skepticism to all things. We do not believe in boogie men and ghosts despite the prolific media supporting the existence of these things. We need objective verifiable evidence. No ghosts, no god, no dice. I'm no fool for dragons and unicorns either. Mermaids "are very real though, they are as real as you and me."
Some people do not believe about anything unless they feel it through their senses and do not recognize what's behind it and some refuse tasks required by their faith, and some find it comfortable to bother thinking about their actions, is it forbidden or allowed. Words I love to Conclude. It is time to find out about Islam. It is time to find out about the truth by using your own Mind. A Time for Thought. A Time for Change. A Time for Action.
( 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)
They haven't nourished their seed. The world is full of hate and that is because of the enemy. Every comment you get saying there is no God, shrug it off! God loves them and they just don't know it yet.