Because they arr rook same
Actual answer: because the Manga/Anime industry is awful, like most things in Japan, and it's all about quantity & speed. Quality is irrelevant. The artstyle is deliberately simplified to the point of mass-consumption (anyone, literally *anyone*, can learn to draw manga-style in a matter of weeks) It's a production line, factory work. Tracing & copy-pasting all over the place.
A few more years and they'll just have machines shitting out the stuff because it's that generic:https://www.thiswaifudoesnotexist.net/