I mean, if you wanted to stretch, it's potentially because Overlord but genderswapped and then otomed. And if you want to get really deep, it's worth noting the games these isekai-ed girls end up are often more likely to be based on otomes/dating sims the jrpgs/mmorpgs, and as we know when it comes to romance, any girl who isn't the MC, the "you go girl" best friend and in some games the single lesbian option, they're all evil for the very reason that they exist and want to steal you man! No matter which man you pick and never mind you've got an abundance of them. And if you wanted to give these stories credit, there's an argument it's commentary on that, the villainess is a villainess by simply existing no matter how normal she is just because she's a girl in the game's MC's way. And I suppose it's worth noting that a lot of villains never see themselves as the villain and if we saw things from their side we might not consider them villains either. And I certainly don't think to count as a villain you need to be outright irredeemably evil, in fact some of the most interesting villains are also nice people and that's what makes them interesting, however, I do agree they should at least do something villainous by the stories standards.
Ooooor maybe I thought too much on that and the real answer if because trends and light novel trends and a lot of light novels are based on isekai-ing the most normal person you could find so the reader can project themselves onto them.