Of course you can always go to professor/TA office hours to get help, but nobody really wants to do that.  If you have a homework assignment due in a couple hours and you need help on a problem, do you really want to email the professor and hope he responds before the deadline, or do you want to walk down the hall and get help from the brother who took the class last semester?
It's all about ease.  Of course, you could do everything a fraternity does on your own, but it's going to be a lot harder.  You can network, find parties, get study help, and find cheap housing all on your own, but it's not going to be as easy.  Saying you could do all those things without being in a frat is like saying you can go powder skiing without fat skis.  Of course you can, but it's not going to be as easy or as fun.