Having said that, in the role-playing club that I am a member of, I've been running the occasional pickup game, usually with the purpose of introducing new players to the game. In fact, I met my group for Dungeon of the Mad Mage that way, and they liked that pickup game so much that they stuck with me. I am certainly not the world's best GM, I don't have voice-acting skills for example; but I do know a thing or two about how to tell an engaging story, and how to stage an interesting tactical combat situation. Even a one-off game of D&D can be fun, but it is hard to find a Game Master for it.

Tabletop role-playing games, except for a few GM-less exceptions, are asymmetrical: The GM carries a lot more responsibility than the players. That translates into the GM having to spend a lot more time for preparation than the players. Often he also has to spend more money: The players just need the player's rule book, the GM has additional rule books and the adventure module. In consequence, although there are on average 4 to 5 players per GM, the limiting factor to the number of games run in our local role-playing club is the number of GMs.

The current pandemic has accelerated a move away from tabletop role-playing games being run on real tables towards virtual tabletops, like Roll20. And finding a GM to run a game for you is still a problem. Especially online, any GM running a game for complete strangers risks those players not showing up, or abandoning the game. So, one thing that evolved over time, was GMs on virtual tabletop platforms demanding a small amount of money upfront for their services. On the one side that covers the cost the GM has on the virtual tabletop, which often force you to buy virtual copies of books you already have in physical form. But on the other side, it works as simple psychology: The player who paid $10 upfront for a place at the virtual table is more likely to turn up than somebody signing up for free.

With 5th edition, Dungeons & Dragons has become more widely known and accepted. So, some people would love to try it out, but have a hard time finding a GM. And "playing the GM" is pretty daunting if you never played as a player before. So GM for hire small businesses evolved. The same way that you can hire a clown for your kid's birthday party, you can hire a GM for your slightly older kids, or for your group of friends. The cost structure is the same as hiring any professional that has to come to your house and stay there for several hours, it ends up costing a few hundred bucks. And if you don't live in a densely populated area, this service might be hard to find.

Virtual tabletops allow GMs for hire to expand their business. More potential clients, no geographical restrictions (other than time zone problems), and easier ways for players to find a GM. And you don't need a group of friends to play either, players can join games individually. So now, there are services like StartPlaying, where a GM can set up a game and sell seats for around $20 per player per session.

I don't know how much Matt Mercer earns as a GM, compared to his occupation as voice actor. But "the professional GM" seems to be a possibility now. And, as an "experienced amateur GM", I'm not sure whether that is all good. Money does strange things to hobbies. Games change when their primary purpose is money instead of fun. Will we end up with professional GMs offering loot boxes in their dungeons? Only time will tell.