One big reason I don't believe in God is that descriptions of the Supreme Being almost always are unbelievably perfect.
Show me a flawed, clueless, unknowing, emotionally infantile God and I'd be a lot more inclined to sign up as a true believer. But then I'd wonder "What's the point in being devoted to a supposedly divine being who actually is no better than us humans are?"
It's deeply suspicious that virtually every religion posits a super-good God. Sure, the Old Testament God and similar gods exhibit some nasty character flaws -- jealousy, cruelty, egotism, to name a few -- but somehow religious dogma manages to put a positive spin on such negativity.
"God works in mysterious ways." "God knows what is best for us, even when it seems wrong." "God sends suffering to those He loves so our sins/karmas can be cleansed more quickly." "God tests our faith to see if we are worthy of Him."
That's all bullshit, of course. Yet billions of people fall for it.
Because religion is humankind's most powerful Wish Fulfillment Mechanism. Whatever you want, religion offers it. Not in reality, but in fantasy. Still, fantasies are pretty damn appealing to us Homo sapiens. We've become so sapient, our big brains are capable of manufacturing concepts, abstractions, imaginings, and such which can become substitutes for really real reality.
Religiosity reminds me of an iPhone slogan: There's an app for that. Except instead of a smart phone application that can fulfill a need/desire, religions offer up a theology.
Afraid of dying? Want to live forever? No problem. Join our religion and you'll spend eternity in a heavenly realm! (Along with loved ones, and maybe even your dog.)
Dismayed by suffering? Perplexed about bad things happening to good people? No problem. God has a plan for you, for me, heck, for everybody! (Details are secret, so just have faith.)
Frustrated by how little you know? Want the truth about ultimate reality? No problem. One day all will be revealed. (Can't say when, but remember that eternity promise.)
Science, by contrast, seeks to know what the universe is like, rather than claiming, as religion does, that whatever you like, that's how the supernatural side of reality is. Since I can't accept that the cosmos revolves around the desires of human beings, scientific knowledge seems a lot more credible to me.
But, hey, if wish fulfillment through unfettered fantasy is your thing, there's an app for that. Just download a religion into your brain.