I think this explanation is a bit of a stretch. Isekais just recycle the same ideas ad nauseam. If it were about creativity, they could do a lot more. You're given a blank slate to create a whole new world and mythology surrounding it, but your protagonist barely leaves the town to explore the world.
Which series does this describe? Guy gets hit by a truck, reborn in a medieval-european-esque world where he joins a guild and does menial tasks like collect plants and kill goblins to level up, but this happens rapidly due to superpower which took no effort to gain.
It's kind of like the MMORPG grind but without the grind. Instead of playing 20 hours/day, you see the character progress in 20 mins/week. The side-characters are as bland as NPCs.
MMORPGs suffer a similar problem: There's so many of them and there's not a whole lot of difference from one to the next.
The mangaka are typically young men who grew up playing these games and not getting out much, so it's no surprise their limited experience of the outside world also limits their ideas.
Perhaps a source of the problem is the business side of things: The studios are basically forced to fit a story into 13 episodes which leaves little room for actually creating a memorable experience. 24-26 episodes used to be the norm but is now the rare exception. The studios don't want to take risks so they'll only adapt popular manga.
Or maybe the consumers just have shorter attention spans, so you make the character go from level 1 to 1000 in the first few episodes to try and hook the audience, then revert to slow-pace.
The iseakais that are genuinely creative (cough 12 Kingdoms) get dropped.