In High School I wrote a one act play for a competition which was a "generic medieval fantasy" story. It was a "hit" and I had a lot of fun staging it, so for the next year I wanted to write another one. But obviously it was time to stretch my wings and conquer new genres as a playwright, and kicking around ideas I quickly realized that I couldn't think of any other genre where I was as comfortable just writing with no research whatsoever. My second play ended up being fantasy but (ostensibly) set in 19th century Europe, and even that specificity required a little research and (more importantly) anxiety about whether I knew enough about the setting.
For whatever reason I think "generic vaguely medieval European fantasy" is a genre where everyone feels like they know the default setting rules (which have almost no relationship to anything actually medieval, but that's a different conversation for a different day) and have fairly similar expectations. This is tremendously helpful when playing an imagination game where players are called upon to do whatever they want and information about the world around them at any given moment may be absent or poorly communicated.
In a setting where players know what to expect they are comfortable driving action. When they show up at a village they know they can go to the tavern for lodgings, and can seek out a general store or a blacksmith (both of which will be happy to not only sell them things but buy their used junk). When some bandits show up they assume they can just kill these outlaws, not need to only fight defensively while they flee to go find some sort of police. You can of course play against these tropes and assumptions, but many of the worst campaigns I experienced or heard about were full of smug npcs telling PCs everything they assumed they knew was wrong.
Now are there other types of settings that work equally well? One would think the real world (either modern day or "familiar" historical eras) would work just as well in terms of assumed understanding of setting rules, but that opens you up to research rabbit holes over stupid details. I've never been in a Call of Cthulu game that did not at some point get derailed for several minutes while we litigated whether X tech was widely available in the 1920s or whatever, and while this occasional hiccup in actual gameplay may not be a huge issue, I think (to many of us at least) it is much more intimidating to design stories set in settings where there might be obscure anachronisms hiding in the weeds or whatever.
Of course, counter to the point I was making about generic medieval fantasy's strength being that people had strong ideas of default setting assumptions, fantasy, more broadly, is the genre of grandiose worldbuilding; of the author being god of their own little universe. And this also, of course, has a strong correlation of why a certain subset of people get into being GMs in TTRPGs. Even though in actuality fiction set in a non-magical real world or real world adjacent setting can involve a tremendous amount of worldbuilding as one develops characters, organizations, and their relationships and history, and even the hardest Science Fiction has potentially yet more worldbuilding, nevertheless fantasy is the genre where the iconic creators are basically all massive worldbuilders and thus people with the worldbuilding impulse natually flock to it.
