The fun part is that they're allowed to assume you're cis, white and straight, but you could be cis, black and gay. If they're wrong, they get off scot free somehow, but god help you if they deem you X-phobic.

They're afraid of definition because then they have to justify a lot of things, namely behaviour, and how they're marginalised, when it's clear that they're not in any way. For example, when someone calls themselves queer, they can pretty much continue living as they've always lived without changing a thing. That's why a ton of straight people come out as queer, because they always bank on the fact that their straight relationship will be assumed to be a straight passing bisexual one rather than what it truly is.

It also allows them to join any marginalised group and get a piece of oppression pie (as if being a minority is a good thing) without experiencing actual oppression. In turn, they get any benefits and protections the minority group have been granted as recompense for decades and centuries of oppression and suppression against them; see grants, literary awards, university scholarships, political positions, etc. They have all to gain and nothing to lose by having vague definitions.

I think the main issue lies in them making previously definite and clear definitions muddier with their gender identity politics. For example, saying you're in a lesbian relationship these days doesn't mean you can't be pregnant from PIV intercourse.