I think it's a combination of things - a lot of things are pathologised unnecessarily, for starters. You can feel anxious without having anxiety disorder, but we seem to have dropped the "disorder" part now and as soon as someone feels anxious, they "have" anxiety and that's part of their identity.
Also, diagnosis takes much longer meaning some people struggle a lot more, and there are people using diagnosis as an excuse. I was diagnosed and medicated for ADHD on the NHS about a year before it suddenly exploded and was everywhere with all the private diagnoses etc, and medication was a game changer - I went from jumping job to job, to holding down a job in the same career for 7 years until different ill health got in the way. What I'm seeing more at the moment is "I got my private ADHD diagnosis, this means I'm unable to work", which I can't wrap my head around. Someone in my situation nowadays would have to wait a lot longer for diagnosis and medication, so it is harder on those people where it has a significant impact on their ability to function.
Social media plays a big role. There are communities for different conditions, which is a blessing and a curse because you can get advice and camaraderie, but humans naturally don't like feeling left out so they think "I'm going to get in on that", and some conditions - neurodivergence, invisible illnesses like EDS, POTS etc, are easier to fake to gain entry to this "club". I do think there are a lot of harmful online communities where the sicker you "are", the more attention and sympathy you get, and people become addicted to that. Like Munchausen by Internet.
Equally, society's patience with neurodivergence has thinned because it's become so diluted with people looking for excuses, so those who genuinely need understanding and acceptance aren't getting it when needed - instead they're told "oh yeah, who isn't neurodivergent these days, that old excuse", and it isn't accepted as a reason for their behaviour.
I don't think it's an issue with a single cause that can be fixed immediately.