I have had the same thought before, and I wonder what the cause is. I think it boils down to basically two things that are somewhat related.
The first being basic marketability. A pretty basic fantasy world like Game of Thrones is a lot easier of a sell than something like Zeno Clash - which really pushes at the borders of the genre.
Maybe a bit more devastating though is that the second aspect is that people just aren't as interested in being really creative anymore. There's something to be said about how nothing is original anymore; coming up with something truly unique takes a lot of energy. So people end up just using existing shorthand to save themselves the time and effort. This also means it's a bit less stressful for the audience in a "Do elephants exist in Pokemon?" sort of way.
If your setting is pretty stock, you don't have to spend so much time explaining it, and you can devote more effort to whatever you really do care about, like character drama, a magic system, political intrigue, Minotaur dick- what have you.
