Imo it’s the same idea as having black people play black characters instead of white people with black makeup. If everyone was treated as equal then it wouldn’t be an issue. However, that’s not reality. People are treated differently based on gender, sex, race, age and so forth. Wanting queer characters to be played by queer actors is a way of making sure they have a space to demonstrate their skill, talent, and potentially make a living off it. Same thing with black people playing black characters, or women playing female characters.

There’s another element, however, in which good acting can’t fully replace personal experience. A queer actor playing a queer character will likely be able to identify with said character much better than a straight actor could, and as such, they would be able to harness their personal experiences and channel them through the character they’re playing.

While my latter point doesn’t refute your point about straight characters being played by queer actors, the former hopefully explains why it isn’t universally applied. I do believe though, that in a just and equal world, things like sex, race, gender and so forth shouldn’t be (dis)qualifiers for any given character, it’s just that we don’t live in that kind of world.