Well, just because the rule doesn't by itself prevent all possible cases of inappropriate cross-rank fraternization doesn't mean it has no value. There are other norms and practices that discourage generals from hanging out with lieutenants, e.g. generals usually get fancy lodging separate from the lieutenants. I suspect that cutting off lower-ranking officers from fraternizing with enlisted men prevents what would otherwise be one of the more common problematic cases.

If the military were even more concerned with this problem, it could have three or more groups instead of two, say, enlisted, officers and super-officers. But there are also tradeoffs to having more groupings, so the military sticks with two (part of this might be historically contingent, maybe three groups would work just as well but everyone is just copying the consensus choice of two).