This is a way for shitty writers to justify infodumping in their story. If your main character doesn't know shit about the world he just got put into, you can justify every other character dumping a huge load of setting and world building down his ear canal. Instead of, like, trying to mix that info naturally into the story, which also avoids the "as you know, John..." trope (where character A explains something to character B that they already should know), but requires effort and skill on the part of the author.
I suspect there's also lots of demand from people that are stuck in the same routine for years and like to day dream that something exciting is going to happen in their life sometime.
The first isekai a lot of people were exposed to was Sword Art Online which, initially, had closer ties to the real world with a scenario that could (in theory) actually happen. Most recently, though, most isekai have abandoned the "it could happen in real life" angle, leading to crazy farfetched stories.
I hate to say it but one of the most recent ones, about the MC coming back as a vending machine, is really good.
It is a wish fulfillment fantasy most of the time, that's why I really appreciate series which go out of their way to deconstruct that aspect of the genre, like Re:Zero or Tate no Yuusha no Nariagari. Especially Re:Zero, which shows what would actually happen if you put a shut-in NEET loser otaku into a fantasy world.
That reminds me of Hideo Kojima’s writing. So much exposition and info dumping in his games. And a lot of it is just non-sense. I think he learned that style of story telling from reading manga. In comics and manga speech bubbles are cheap while drawing action scenes is expensive. hence the over use of exposition in manga. Lots of Japanese writers think you can copy that style of writing for the screen. Even Japanese live action movies are often like that.
Something really annoying is that they make an isekai, in it they could make anything in their new world, right? 95% of the time it's the most generic, cookie cutter mmo fantasy world with levels and game mechanics.
Another things is when they spend sometimes multiple chapters explaining the most obvious things, like there's an adventurers guild, it had the same rank system as any other isekai, they probably get their power level measured, etc etc.
It makes me sad, because there are good isekais, but most of them are just really lazy.
To be fair, the main appeal of isekai is a person from our world having to adjust to suddenly living in a fantasy world, and applying their knowledge from our world to the fantasy world in some way, which is completely lost if you just make it a straight fantasy. I do agree that there are way too many isekais, most of them copy/pastes of each other, and there are other ways of setting up a similar scenario that don't just use the default isekai setting. Kekkai Sensen for example is a good example of the 2 worlds merging trope which often allows for a lot of the same sort of story beats as isekai but going both ways and with a lot more flexibility.
In my experience this only lasts for the initial arc at best and then the protagonist becomes fully integrated in the world and is no different than someone already living there, with all subsequent arcs receiving no benefit from having someone from another world. The vast majority of isekai make no effort to integrate the protagonist's experience from another world into the plot and instead rush to sweep it under the rug.
There are of course exceptions to the rule but the vast majority of isekai I come across is as I described above.
This is something I have a real love/hate relationship with in Overlord. In the books, we get to see a lot more of what's going on in Momonga's head and how his experience as an average salary worker affects his decisions in his new role as an all powerful lich and the dichotomy of how he perceives himself vs how the people of the world perceive him, which is really cool and fleshes out him as a character and the world a lot more. But then people in the new world will casually throw video game terms into their speech like "putting levels into x class," and it really throws me out of the world.
Yeah 99% of isekais suck and don't use the genre to it's full potential in that regard, but that's what the general appeal of the genre is supposed to be to most people and why they might choose it over a standard fantasy. Most popular/highly rated isekais do usually utilize this aspect to a reasonable degree afaik.
Honestly isekai really only covers the first chapter or so. After that it's just another {progression} fantasy. It's a way to shortcut the Campbellian structure of these types of stories. You skip past the call to adventure, the initial rejection of the call and the hero leaving their old life and go straight to the new, unfamiliar and dangerous world.
What's works is when you have an Isekai that absolutely doesn't shortcut it, but actually ties the initial rejection of the call into how they felt about themselves in their old life
There's one I like called Tanya the Evil, where a guy gets pushed onto railroad tracks by someone he fired and gets isekaied to a fantasy WWI era world. He then becomes the worst person he possibly can just to spite God XD
That kinda exists though. e.g. Devil is a part timer, Jahy-sama won't be discouraged. Though they don't entirely transform into regular nonmagical people; but I bet that also exists somewhere - there's an incredible amount of mangas and light novels.
I used to hate isekai until I learned about slice of life Isekai which is peak fiction such as:
"Uncle from another world"
"Isekai Izakaya (Japanse pub)"
"reincarnated as a vending machine"
"Restaurant to Another World"
"Ascendance of a bookworm"
etc, etc, and on into infinity. If you have something you like as a slice of life series, there is probably a pretty okay isekai version of it.