I asked this question myself a while back and didn’t really get a satisfactory answer. I get the feeling this is the kind of thing that comics fans accept as “just the way things are/have always been” without questioning it too much.
And don’t forget, Batman is on the Justice League AND The Outsiders, too. Plus, he’s in however many Batman limited series DC has running at the moment. And during the '90s and '00s there were usually 3-4 or more Batman books running at any given time, in addition to “Batman” and “Detective Comics”. It’s just one of those things. 