Why can't anyone just be from one of these fantasy worlds anymore? What's the obsession with having an MC who knows what a smart phone is in a world with elves and magic? It's not enough to be a fantasy hotel owner, you also need to be a clueless dipshit about the world you inhabit as well...
It's called isekai as noted, which is an insanely popular subgenre right now. Also, one of the longest written and most popular isekai works is a self-published serial novel called
The Wandering Inn.
So that's really why. It's just capitalizing on the popularity of isekai. Now, whether or not you like isekai is a different argument as a lot of what we call isekai today is pretty rubbish.
As to "why this narrative mechanic," there is no reason, not any more than the MC being the chosen one or happening to have a bunch of perfectly placed girls all fall in love with him, or whatever.
On a slightly off topic note, isekai is a more dystopian take on portal fantasy like inuyasha, escaflowne, chronicles of narnia, etc. In the old take, there was a reason for the protagonists to want to come back, and they do. In isekai, the protagonist is generally better off in the new world, doesn't want to come back, and often can't come back anyway (i.e. they died in the old world).
It's basically self-insert escapism cranked up to 11.
Correction: it only makes zero sense if MC is supposed to be 18+ and clever enough to run a hotel. If MC is a barbarian kid who came down from his mountain village for the first time, he still needs plenty of explanations. There are also options such as "I dropped out of a portal yesterday and remember nothing" and a protagonist who actually talks to the audience and explains things for their benefit.
Conventional RPGs and fantasy novels have used all of these successfully. But I guess shota barbarians are underrepresented in porn games, even if Monster Girl Quest has one.
Read above basically. It's just a popular genre now.