Self-selection of confident, motivated men is a part of it.
Membership/leadership in a fraternity is all about social contracts. You come to college with 0 or maybe a few strong social contracts, and students fail/drop out due to lack of support all the time. Fraternities provide 30-60 strong social contracts freshman can rely on.
Fraternities also (by way of current major makeup) select for certain majors. I was a highly involved business major, and recruited many other business majors into our chapter. We had 1 art major member.
The mission is to make you a better person. Most buy into that and do it. This leads to higher income.