I took public transit from and around impoverished areas for years in college, and can count the number of incidents that I have witnessed on one hand. It's not 'that' common, it's just jarring when it happens because it completely pierces the artificial bubble you can place around yourself in society.

That being said, after years of driving the amount of absolutely batshit insane stuff I have seen I have lost count (though I have been driving longer). It's just that you pretty much still always have the car cocoon around you.