why

Maybe the same reason some girls would rather not date than go to prom with the "class geek", especially when their friend is going with the Homecoming King.
It's usually having to do with a girl's own social expectations and tent talk or what she has heard about other groups. The girls who don't have mothers and friends who are into that don't care so much and they will just get to know the girls in the houses before making their choice. And then they appreciate that there are many groups who offer a sorority experience that any girl would envy. But to others, their ego or the family ego just wouldn't allow it. It makes sense, but it's sad for them because they end up missing out on sisterhood of any kind. The groups at UT really are filled with wonderful and beautiful women. So it's a shame, but it happens.

It is what it is and does not require your "belief" in it to be true.