Basically in the 70s and 80s there were so few venues that the folks who ran the venues called the shots in terms of costs, dates, etc. They paid venues to run a show there. What choice did they have, there was maybe one venue for hundreds of miles in some cases.
Sometime later new mid sized venues started popping up all over the place. Venues run by cities and etc (who had built these places) were operating at just about break even, there were lots to choose from. Venues sometimes paid up front for a show and then would take a small cut of some other activity on the day of the show. The tables had turned, for a while.
Now venues are run by the folks running the whole thing, the venues, the tickets, all sold by the same people.
What never seems to get mentioned (often enough) is people will pay these crazy prices ...