frats and sororitys

I'm curious I've noticed a lot of houses only go to certain houses/ don't go to some houses at all. Why is that? I.E. from what I can gather on this site "top house" sororities only party with top house fraternities. Why doesn't this ever change? Like will a top house freshman girl ever go to not a top house to try it out? Or does she just assume they aren't worth her time?