Many Reasons Why
I learned long ago to respect those that don't believe what I believe. I remember the first time I realized that believing in God could be a choice. Granted, I am not religious and never have been. The only time I went to church was on Wednesdays in my Catholic school and I lied to the father there at every confessional (we weren't close like that so he didn't need to know my business). I had had a lot of friends who were of different religions. I had a friend who was Hindi when I was in first grade (who also went to the Catholic school), so I became accustomed to meeting people of different faiths early on.
Atheism didn't become something I was fully aware of until I was around seventh grade. Before then, God was always at school. People prayed on tests, prayed for pizza, prayed for the teacher to not check homework. We had religion class (though I never really paid attention) and often had to pray (or just look at the desk in my case) before class started every day in the Catholic school. I had gotten an inkling that God was not always the greatest force in the Catholic school (ironically) because that was when slavery, the Holocaust, and the murder of Christ all became part of the curriculum.
I don't remember the first atheist that I met, but I do remember reading Night for the first time and understanding why someone would want to stop being religious. But, I know that horrible acts are not always what drives someone to not believe. People want proof. Granted, most of the people who want proof are often always running in circles because their evidence gets disproved daily, yet I've met many people who have said that there's no evidence that God or anything the Bible/Qur'an/Torah says God did is real.
That argument drives me insane for two reasons. For one, the Bible was written in a time where meat grew maggots on it, the earth rotated around the sun, and sperm had a little person in it that was sent into the woman to incubate. To say that the Bible is not factually accurate is like saying that the Odyssey is factually accurate. Sure, there were things that may not have been correct, like the seven days thing, but if a day became 24 hours between 147 and 127 BC and the Earth is 4.5 million years old, who's to say how long a day was back then? I believe the Bible has accuracy, especially with bored scientists investigating it and finding evidence that biblical events did occur worldwide, but I don't think that everything matches with today's science any more than I believe that Jesus was white. (Just look at the geographical location).
The second, and most important reason, is that science isn't always right. I met a guy recently who does work for de-extinction (which is pretty self-explanatory but in case you don't know what that is, it's recreating extinct animals using old DNA and a surrogate from a similar, living species) and while he was fairly interesting, it was strange to see how working to reanimate species makes him think. For example, he mentioned bringing back the woolly rhino, which I guess is like a rhinoceros version of a woolly mammoth. While it looks cool, to recreate it would make no sense. The ice caps are actively melting and we are already talking about polar bears and penguins being homeless. Reanimating another polar animal would be idiotic, even if it could eventually reshape the terrain because it is more likely to die as a juvenile (because a warm-living rhino can only teach it so much) or starve to death.
As a person studying science, it's interesting to see the growing interest in what humans can do. Similar to the good old days of tiny humans in sperm and maggot meat, we have put science on a pedestal as it taught us that everything that people thought they knew was wrong. But, in a way, it wasn't. Those people were just observing their environment and making 7 and 4 into 74. Before I get too off-topic, science isn't an answer to everything. Sure, there are answers. Answers that are valuable, like that humans need both men and women to be created (perhaps that was why masturbation was frowned upon...) and that if you leave raw meat out, something will lay eggs on it. But, science isn't always the answer, as can be seen by the many mysteries of the world like the female lizards that seem to partake in binary fission or the trillions of unknown creatures in the ocean and in space.
Science-fuelled anger aside, sometimes people just don't see God working in their daily lives. As an avid player of Sims, I can say that it is hard to be God. Sims is the closest one can get to being a god and seeing their creations happen. You have total control over a set of beings. You can make them do anything, you can take their free will, you can make them gay or straight or purple of fat (not short though), anything within the set height limits is possible. I have loved it since I was a kid, and Sims is the reason why I am a rather likable, go-with-the-flow person. Mainly because I learned the hard way that controlling everything leads to stress and heartache.
I had no intention of getting Sims 4, as EA Games already owns a lot of my soul and money. But, an offer I couldn't refuse (three games for $30) came up, and I ended up buying into the Sims 4. The difference between the Sims 3 and the Sims 4 is huge. For one, the sims are more realistic and what they don't have in personality, they make up for with awesome new graphics and a realistic level difficulty. As a successful Sim town owner, I know that the way to ensure longevity is to have a lot of kids every generation that will eventually get together and make more kids until there are basically only two large families (then we run into a cheetah situation).
On Sims 3, this is easy. I meet a person that has a few of the same traits, work a little magic, and soon they are married/at least in a relationship, expecting a little one. Sometimes, it's even easier than that and all you need is a potion and a kiss. In Sims 4, it is way harder because their moods fall every six seconds, the things they want constantly change, their paths to achieving their lifelong aspiration are unnecessarily complex... It's a whole thing. The reason I bring this up is that if God is anything like that, there's no way you would know if he's there or not. If Sims started getting cancer and trauma and having their houses hit by lightning, no one would play it. It would be impossible to help everyone, and they'd feel bad for picking favorites, especially if they were forced to make everyone and everything essentially from scratch.
At the end of this whole thing, I will say that I can't tell you what to believe. Obviously, my beliefs have developed over time in a lot of different ways and to try to make you see where I'm coming from would be taking you down a whole journey of awful elementary school kids, obsessively playing Sims, and years of science classes and frustrating conversations. No one would sign up for that if I paid them to. I can say that I have been shown religion in a series of ways and you've been shown what you believe in a series of ways. I can also say that you not believing doesn't ruin anything in my life. I've learned to live with others that have differing opinions from me. But, I can say that whatever religious journey you're on (knowingly or not) doesn't end. So, good luck with yours.