A few things.
Work vs hard work. No, running games isn’t that hard. But it is work. A lot more work than most players realize. A lot of referees also make things harder on themselves by prepping the wrong stuff, making assumptions about what the PCs will do, etc.
Skill level. Just because anyone can, and yes anyone can, does not mean everyone’s equally good at it. Take any sport as an example. There’s usually a massive difference between amateurs and pros. Not that paid referees are closer to pro athletes in skill level. But the perception is there. “This person must be good since they’re charging.” That is not always, and often not, the case. My one experience with a paid referee was middling at best. The group had four longtime referees as players and any one of us could have done just as good if not better. In this case the “pros” are regular referees putting out their shingle.
Shifting baseline. When most gamers came into the hobby prior to the 2014 D&D boom, we were introduced by a friend, older sibling, uncle, teacher, etc. They had experience with the game, sure. But they were not “pros” at running games. The people coming in with the 2014 D&D boom mostly discovered or rediscovered D&D while watching professional actors run and play in games. Whether Stranger Things or Critical Role. Your cousin who ran Keep on the Borderlands for you back in 1984 just does not compare.
So you have kitchen-table referees being compared to professional actors and most people are noticing the vast gulf between those two groups. And surprise, people come along to fill that gap. For money of course.
Thankfully a lot of other people came along and put their advice out there for free with the option to buy it collected in a nice book. Matt Colville. Mike Shea. Kevin Crawford. Justin Alexander. The majority of referees in the OSR/NSR scenes.