If I had to guess: It is more "immersive" to in-universe pay for what is effectively a whore in your fantasy world, I think some sad sacks actually enjoy the idea they're paying if the whore is good because it makes them feel like they're stuffing some $100 bills down some cat girl's g-string or some weird shit because they're too poor or chicken shit to do that irl. It is like you can pretend to be some rich guy in a suit and get some high class escort to walk around with for an evening if your close your eyes hard enough. RP is a weird fucking world, especially this sort of stuff. If you got a good show, people will depart with their monopoly dollars to go for a ride, and some are so desperate for such an experience they'll spend some real money for their monopoly money.