At a mechanical level isekai in webnovel form lets you introduce the reader to a new world much more quickly and easily than if the MC were in that world with no reference to our own.
It's why I think it's so popular, especially when coupled with a "System" as in litrpg which accelerates that even more.
It's easy on the creators primarily, they get to skip a lot of steps and don't have to explain away things like cell phones.
For consumers, they get quantity.
Although it shouldn't be impossible, it seems really hard to get any kind of real quality out of isekai stories. Usually the stakes are low and the setting is surprisingly similar (even though having a different setting is ostensibly the entire reason for the genre). All you end up with is variety in characters, and even then there are common tropes.
As you said, the readers get quantity and honestly above all else that's what they want.
They already enjoy the stories enough to buy them or support patreons for them, I don't think trying to up the quality would be wise for the authors assuming it slowed down how quickly they wrote.
I used to dig that explanation, but even outside of the ultra-pulp areas, every fantasy world is ultra samey and does not really need to be introduced to the reader at all. It's either a tolkien derivative or... whatever the most influential xianxia novel is. I'm not as good with those, but this one could not help but notice that all the cultivation books are cut from the same cloth.
And don't forget how easy it is to explain your book's premise. Most stories have trouble marketing what makes them special. "It's a young adult action adventure that gets very into describing food, with teens going on big journeys and discovering secrets, with a whole lot of lore and stuff, and we see multiple generations over the course of the books, with characters going on to becoming the myths that later characters learn about in other books, and also it's all animals" is hard to explain in five words. Then take something like "Isekai but it's a Roomba." There, I told you what it's about, and you already know with near 100% accuracy whether or not it's for you.
It's why I think it's so popular, especially when coupled with a "System" as in litrpg which accelerates that even more.