selection bias. people who know that what they want to do is wrong but don't have the strength inside them to not do it just because its wrong tend to gravitate toward an external authority that will dictate and enforce their morals for them. they also, like all of us, believe that they're more or less average, so they think that the dark shit in their heads is what everyone always experiences. They think that because they need god to bully and extort them into being decent that anyone who doesn't believe in god cannot be decent.

To quote Penn Jillette (from memory): "They say, 'Without God, what's to keep you from raping and murdering all you want?' And I respond that I do rape and murder all I want, which is zero, and I wonder how much they want to rape and murder that it's the first thing they think of when confronted with the idea of freedom and responsibility."