It's cultural. In Japan showing pleasure is seen as dirty and shameful, so the ideal Japanese woman is one who is flustered from being brought to the edge.
It's stupid, but that's how this goes.
Makes sense, also may explain personality switch from smug and predatory to shrinking violet the moment they are at the receiving end.
There's also an aspect of an artistic game of telephone that happens in the manga community. One hentai artist learns by copying another hentai artist, and their stuff gets copied by other hentai artists.
It would be nice if just once, right before climax, the other girl would say "Uhhh, sorry, don't want to make you feel uncomfortable" and walk out the door.
