@maxmustermann Because the premise makes for easier writing.

Self insert loser protagonist gains unfair advantage right off the bat, wish fulfillment for many NEETs.

Many aspects are borrowed from Japanese RPGs, reducing the amount of explaining needed.

World building is easier, as characters spout exposition at the audience under the guise of explaining things to the protagonist.
Something totally illegitimate to do if all the characters are supposed to be familiar with the world.
If it wasn't an Isekai, you would have to "show, don't tell", but that's harder to pull off.

It's a lazy genre.