its because the majority of them are shit writers, I mean just look at the isekai genre, manga is filled with alot of copy pasta of just bad troupes/stalling because they can't think of something interesting to continue the story line OR this is me 100% being ignorant since I'm not a woman but maybe there is ALOT of toxicity in the lesbian community, someone could correct me but yea it has to be one of these 2 reasons because it just happens too much, someone cock blocking the Main pair because jelly, trauma, hidden closet hate, or a toxic/horrible ex, these troupes are WAY too common in yuri manga and I legit can barely think of any yuri series without atleast 1 of these troupes sadly