Because they are undisciplined. But who wants to live with rotten kids? Why don't their parents discipline them? Because disciplining stinks. It is much easier to just clean up the mess yourself than it is to force the whining, lazy 4-year-old who made it to clean it. Disciplining is not just unenjoyable, but ungratifying. You don't see your kids improving, because they are growing and changing constantly. For all you know, all those battles of the will over whose job it was to put on socks were totally wasted; she'd have grown into putting on her own socks eventually anyway.
Furthermore, no matter how many times you tell them to chew with their mouths closed and say thank you, they're still going to horrify you with new heights of rudeness when you take them to see Great Grandma. So why bother?
Every day here feels like a bad day. Someone is always stuck in a cycle of bad moves. Someone always gets yelled at. Someone always ends up crying and neglected while I'm dealing with someone else. I've only got one fuse, and it only takes one kid to burn it down for all the rest of them.
You see so many rotten kids because all kids have rotten parents.