"Why must yuri mangaka draw out the relationships!?"
I think you mean every mangaka, and every type of relationship they draw and write about. As for why, well it's practically traditional Japanese romance storytelling by now I guess. That or they're forever alone and have no idea how to write about a happy couple that's already together? I don't fucking know, I do wish there were more stories where the whole thing isn't just the lead up to the confession though. They're rare but they're out there and I really need to compile a list of the ones I'm already aware of, if only to rescue other readers out there from sinking in a sea of drawn out will they won't theys.