Firstly, they can't understand. ****, I can't even understand my own OCD until its active, then its agonising, I can't expect others to. Secondly, they don't know how the handle it. The only person I have ever trusted enough to understand my issues is my psychologist, and I don't even fully trust her to understand it all, she deals with it right about 60% of the time and she has years of training. When I tell other people, they typically make things much much worse, because they just offer infuriating advice such as "just stop thinking about it" - "I can't its OCD" - "you can". Fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuuuuuuuuuu.
So people just don't know how to deal with it, typically. They do tend to care, in some way, they just aren't able to help.
And if they don't want to help, they don't want to hear about others suffering, because hearing about suffering ****ing sucks. If they can't do anything about it, and it sucks to hear about it, then yeh.
People however do think that mental health treatments are good, and adequate and actually helpful, when in reality its all pretty horrible, esp in the UK. That sucks, because they will go all in on mental health week, but being aware of it doesn't amount to **** when the crappy ****ing government budget £5 for mental health services, and all of that is CBT that is about as effective for helping mental health as shouting at the patient "you didn't try hard enough, ****" (which is what IAPT CBT ends up being).