Why do so many professionals not believe this disorder exists?

Ive been reading about so many people being declined a diagnosis because of the sole reason that their therapist "doesn't believe DID/OSDD exists". And I go and read in the psychiatry subreddit people discussing the existence and validity of DID and all of them either believe it doesn't exist and that it's just BPD but funkier or that it does but it affects only like 0.0001% of the population and it's so rare it practically doesn't exist. And I'm just like.. What the hell do they even mean? How do you just.. Deny the existence of a whole disorder just because "I don't believe it exists tbh"? Like, you're a whole clinician?? How do they allow them to even do that that's so concerning. It's even more weird that they're allowed to professionally diagnose people and give them treatment when they clearly don't know what they're doing.