Note that during our early existence, we were part of much smaller groups with greater blood ties in common. Then again, some behaviors may arise like emergent properties, where it is not a simple task to dissect all the contributing factors. Given the complexity and flexibility of the brain, it may not be so unreasonable an possibility. I'm not sure what explanations have been attempted or dismissed. They have no umpire, no rule book.
Some arguments are better than others.
Remember, it's not the number of believers that is important but the validity of the belief. Mind you, I'm not suggesting that the number of common beliefs is not something to consider, just that majorities can be wrong. After all, some of these participants have been engaged in killing each other, in part, over religious differences. As I mentioned previously, observing accountability happening in this lifetime would go a long way to convincing some that moral judgments are meted out by God. You're a piker.