Doesn't it have something to so with making their mouth look like its more a part of them rather than just a line drawn on them or something like that?
It's a cheap way to demonstrate closed lips without actually drawing any lips i.e. muh minimalism while saving on budget.
That's not even the worst of it, though. What really grinds my gears is when they show girls standing with their feet facing each other like they're fucking two and don't know how to stand properly yet or are pissing themselves and are desperately trying to hold it.