Because we are evolved animals that don't have supervision, and we make our own rules. We actually have learned to set ruls for ourselves to help function in various types of social organizations. It's that despite our ability to reason we still have a primitive emotional center and it drives us to make very bad decisions at times. The book Emotional Intelligence explains this, and offers methods to manage our emotions so they don't become hindrances and destructive.
Evolution explains this. Oddly from an Abrahamic assumption this is how God made humans. This only begs more questions as to why a God would deliberately create humans with such conflicted brains. Some believers claim it is to challenge us. Really? When so many fail, and so much death and destruction comes from it? Either the God was playing dice with the universe and didn't know the result, or it knew what would happen and did it anyway. Either way, and ignorant God or all-knowing God is on the hook morally. I haven't even mentioned mental illness and birth defects. Theists trying to claim a Diest God can get away with this. Trying to claim an interventionist God? No, there is too much to justify and explain away, and the God can't escape being a monster.
Right. Good people will be good theists. Bad people will be bad theists. Religion doesn't make bad people good. Atheists know their morals can't be hidden behind belief in a God so have to be more introspect and responsible.
Yet it has mixed results. For all the promise religion offers it all comes down to how decent the believer is, and there is a history of abuse. Again, no supervision. It's clear many believers don't really think a God is watching and they can do anything they damn well please. They might as well be atheists, but they can excuse their immoral acts as doing God's will. And others buy it.
I would be impressed if the vast majority of theists acted with wisdom and high moral principles in a way that can only be explained that they have tapped into some wavelength, or essence, or power, or influence, etc. But we don't. The good people who are believers is better explained as just good people, just as atheists who are decent, loving, moral people are good.
Historically religions were the first type of social contract, where the authorities had to organize and manage permantent settlements, and they used gods as the ultimate authority. This is explained by sociologists, and they can see how early humans created gods for the sake of authority over the people. As we know this has had mixed results, and the eventual response by civilized people was to eliminate religious authority and create secular governments. They couldn't trust religious authority due to the competition between religions and the abuse.