It is not a "can't" thing. There is no legal reason that we can't do it. It is a "doesn't" thing. We just choose not to. Nobody has ever come out and said why, but I assume the shorter length of the event, the sometimes confrontational close quarters indoor atmosphere, and the lack of lobby space to put in a separate concession facility for beer are the reasons.
It is irrelevant to me one way or the other.