The reason is because we (society I mean) keep rewarding assholes for their asshole-ish behavior, so it becomes the culture of "leadership". Nobody remembers what leadership is, they just think being an asshole is some kind of requirement to achieve it. The world ends up run by assholes (and take a look at the world today, and it's clear enough this has already happened).
Assholes didn't just end up in charge, people put them there - and as choices go, it's totally self-defeating. The people who put them there always act so surprised / disappointed too, as if being betrayed and ruined by the asshole they rooted for should somehow be unexpected. Of course they fucked you over, and everyone else they could too - they're assholes, that's what they do !
First Step: Stop being impressed by assholes. Stop being impressed when someone gets away with being an asshole. Stop supporting the asshole side in any conflict with non-assholes. Stop forgiving assholes for their actions and forgetting what they are.