@sjmcc13 The fact is, many of the early Isekai model stories we see today and SAO (which had a huge influence on this group of stories) takes elements from RPGs.
And these Isekai stories are often taking elements from other Isekai stories. The Demon King, Hero, levels, etc. are being taken more from the influence of Dragon Quest (which is pretty much the biggest thing is Japan; so big, they had to make the games have a special release date just so that people won't skip their work or school to get the games).
It's like how, if you use Naruto as a reference for your series, you are indirectly taking reference from Dragon Ball and Yu Yu Hakusho...
Dragon Quest is pretty much the largest thing in Japan and these Isekai stories, directly or indirectly take elements from it. As such, they are RPG fantasies.