Every school is a little different so every frat/sorority doesn't have chapter in every school. Some schools don't even have frats/sororities if the admininisters think they are too much of a liability. There are no big bouncers at private parties to check IDs. The police usually doesn't come unless there is a complain so most can easy get away with undersage drinking. And because it's like the first time most kids in an US get an opportunity to drink as much as they want, a lot of bad things happen. That's why while not all frats/sororities are bad, the group in general as a whole sometimes has a bad reputation.