it's not just the Japanese that do that. it's literally just the male fantasy - to have a bunch of young hot women fawning over him.
considering the target audience for isekai manga is primarily males since most isekai is of the shounen genre. the shounen genre target audience is young men typically teen and young adult males between the ages of 12 and 18 (although really all the way into their 20s these days since modern men are more immature than they were in the past).
Did u even read properly what i said? I said women who look like children. Wanting to be flocked by hot women is normal, because who wouldn't want to? Even i want to be flocked by hot guys. That's normal. But fantasizing about children?? Now that's wrong. Manhwa and manhua doesn't normalize that sht where children is over sexualized. But maybe there is some, but you can count it in your hands, unlike in japan there is even a word for it "loli". That's the difference between the three.

I dont really get what's with Japanese authors putting older men into isekai and pair with with a girl who way younger than themlike bro already have the mind of a 30 y/o and he becomes an elf who lives more years with the mind of an adult btw and even saw her as a kid and teenager. Like we could have just a found family instead of romance