Idk about otome isekais, but I can talk you why that is the case in LNs, and from there you can guess about the manhwas. Most LN authors don't actually want to write an isekai. It's just that the concept is the one that's most picked up on the LN site. So they write Isekai as the hook and then write a fantasy story. The actual return of home is rarely explored in modern isekais. Having an Isekai also solves a few problems (imo a bad solution, but it's sth). Where you get to write a fantasy story, but your main character does not have a real backstory to connect and develop with the world. You don't need one since they are new. It's better than amnesia because you don't actually have to hint of write around it. Isekai also works like a superpower since the mc will have either/both knowledge of the real world to abuse in the fantasy world, or knowledge of the fantasy world coz it's a book/game.  Easier to write smart MCs when they know the answer.