Mario Kart generates massive revenue for itself and its participants
It's often joked that there's no way that Bowser could possibly afford the manpower, real estate and materials for all of his castles. But what if he actually can? Mario Kart 8's replay-sharing system is dolled up as "Mario Kart TV" and advertised on boards on the tracks in-game, and there is a track called Mario Kart Stadium--not Wario Stadium, not Mario Circuit At Night With Fireworks. It's a track that is just about Mario Kart.
So if Mario Kart is itself an organization, then that organization getting richer and richer explains a lot. It explains each Mario Kart game's tracks becoming steadily more elaborate as time goes on. If the characters buy into it and take some of Mario Kart's money to build racetracks for mutual profit, that explains a few things as well. Bowser would have an awful lot of castles lying around, especially after Super Circuit, and Special Cup in Mario Kart 8 is almost all him. An airship course, a desert course featuring his skeletal likeness, and then the token Bowser's Castle course.
This would also explain why sometimes the theme of the track and the character it's named for don't match. Take Mount Wario for example--why would Wario want to build a ski resort? Wario likes gaudy tributes to himself and plastering his face on anything he owns. But what if that's his plan? He uses his gains from Mario Kart to buy a mountain to serve as his race track, runs a ski resort on it during Mario Kart's downtime, and the money he gains from those will go into a fund to carve his visage into the cliffside, rushmore-style.