I literally couldn't care less about demographic checkboxesThen you are not the audience those "demographic checkboxes" are there for. (Neither am I.)
I'm a white male engineer, and I've never had the experience of going to a tech conference and wondering whether I'm supposed to be there because I don't see anyone else who looks like me. That experience is common for engineers who are women, POC, or in various other demographics.
For all I know you might be a member of one of those demographics, but if you "couldn't care less" then I assume you haven't had that experience either. That doesn't affect your right to an opinion, but it does mean you are not fully considering at least one reason that demographic diversity is important.
Ideas are what matter to hackers and engineers.
I haven't met a single hacker or engineer who wasn't also a person, and other things that matter to people include feeling welcome and accepted.
If you're accustomed to feeling welcome and accepted at technical conferences, it's easy to say that feeling welcome and accepted doesn't matter to you at such conferences. If most conferences didn't make you feel that way, it might matter to you more to find one that did.
One way to make people feel welcome and accepted is to reduce the extent to which they feel like the "odd one out", in as many senses as possible.
As autarch points out above, this isn't an argument for accepting any old talk so long as it fills some demographic quota. It would be a strange conference that replaced quality of ideas with demographic diversity in their talk selection criteria. Rather, it just means conferences have more than one criterion to optimise for. Multi-objective optimisation is complicated and involves tradeoffs, but "all or nothing" is unlikely to be optimal.