Finances. That seems pretty valid to me. Cheaper to stay where he is than to set up a completely new residence.
I can't see that it makes any difference where he chooses to spend his time whether it be at his GF, a bar, a friend's house, the park... He might even be sleeping over from time to time -- or even regularly. So what? He's actually doing you a favor by being gone a lot. Neither of you have to put up with the other one.
So, since he isn't there alot, what difference does it make to you if he moves or not? Sounds like you're being a little unreasonable here. It is his house, too, and it's where he has chosen to live. (legal term here--legal residence)
Focus on what is legally relevant, not morally indignant or petty.