I just skipped 98 posts to post this answer. I'll read more later.
They don't believe in him mainly because he never did anything. no phonce calls, no letter, no memos... God is simply non existant to the atheist.
If I went just by my answers to why questions, I'd be an agnostic, too. Why suffering, why does God not speak to everyone, etc.
I don't. I read John's Gospel, got fascinated with the man Jesus, had several unusual incidents happen to provoke belief, and then told someone I was ready to believe and the world changed. That's me. Based on studying the teachings of this Christ, here's my answer to why God is "non-existent" to the atheist.
From the beginning, according to the religion that Jesus' apostles passed on, God has always chosen a person to build a nation with. God would be God to that nation, and through them reveal himself to others. They fail, and God looks bad.
Seems dumb to me, but I'm not God. Most of Christianity is a counterfeit religion having not much to do with what Jesus' apostles taught or their churches lived during those first couple centuries. The rest, that's not counterfeit, is rare and notoriously unsuccessful.
Those first couple centuries, though, they were not unsuccessful, but very powerful, and those who encountered them were quite ready to acknowledge the power they had with God.
I don't know why Jesus doesn't have more such churches nowadays. I feel like where I live, people are impacted by the church, and those who encounter us don't get to be the same anymore. God touches them.
Why he does it like that, I have no idea. Sure looks like it's true in experience and application, though.