Even though university students are adults, they often act like high-schoolers or children. Going to college, they have to create a whole new social network and meet new people. Fraternities and sororities make it a little easier to adjust.
Some of the frats and sororities are social, but some are also for pre-law and pre-med students. They are good for building networks, which is important if you want to have a successful job.
I think the frat and sorority presence varies with each school. At my college, for example, the rate is around 15% of students joining. A friend of mine in another school says it's 50%.
Joining a frat or sorority takes time so maybe the Ivy League kids are just too busy with work, lol.