A good amount of the lounges out there will already let you pay to get in.
I would guess that real estate at an airport is way too expensive for somebody to come in and open just a lounge and be able to make money with it. The airlines are already leasing or whatever quite a bit of space for their operations, so the cost to set up a lounge is much less for them.
It's also possible the airlines have it in their agreements at some airports that they can not lease terminal space in their terminal to a lounge that competes with theirs.