The same reason every story has to be isekai as well. Speedwagon. No, wait, that's a jojo reference... I mean bandwagon. They want to grab some of those bottom feeders that hunger for cheap, unhealthy food that will kill them.
There's no reason to not just outright write a "princely", "male-ish" or tomboyish female character and give them a somewhat believeable backstory as for why they like girls instead of going 'hurrdurr got isekaied into a vagoo, time to fondle muh boobies, hehehe" every time...
They literally cut their main characters short. And no, that was no castration joke, don't be a dick.
