A sealed can provides someone who decides to drink some protection from tampering. And these common sense reasons go on and on. Everything you mentioned about liability was wrong. First, whether alcohol is consumed out of a can, a keg, a punch bowl or any other conceivable container in a fraternity house, sorority house, dorm or any other living unit, and something happens to that person who is doing the drinking, the proprietor of the living unit is liable. The entire national organization is also liable.
Liability exists regardless of who supplied the alcohol, or even where it was consumed.
If a underage student consumes alcohol in the dorms and then comes to a fraternity party and falls off a pool table while dancing, and cracks his head open, as did happen earlier in the year at a fraternity house in Bloomington, the fraternity is still liable. The University banned kegs to avoid lawsuits.