It's just your basic "us vs. them" mentality. Humans are wired for this (as are other animals), so just about any arbitrary groups you put together will develop this mentality over time. You can even split the groups, mix the members and make two entirely new groups, and eventually you'll have "us vs. them" again.
You see this in sports fandom, in politics, in nationalism, in.... everything.
I pledged a really, really down-to-earth fraternity. Great guys, but no different than any guy in a dorm room. Same people, different living arrangements. That's it.
Were you at UNL? If so, what frat, if you dont mind saying?
I think lots of times people that didnt pledge dislike the easy access to parties that frats have. I mean, you are on the campus, you have the girls houses usually right down the road, you have all this space in a huge house and girls flock to the house parties, not because of the super cool guys in the house but because it is close by and you have free beer.