I think "believe" is maybe the wrong word. I've dealt with a few medical professionals and I've come to the conclusion that they are mostly just like you and me, unsure of what's going on, but armed with more helpful niche knowledge and the pretense they know everything. They have come up through a process of learning that leans heavily on the scientific method. Shit needs to be verifiable. That is not to say that none of them act on hunches. But these are mostly hunches they have a body of experience for. Many psychological conditions are hard to diagnose. They can't check the liver stats or x-ray the ankle as an indicator. They also know mental care is underfunded and understaffed in most places. So I don't think it's a question of believing these conditions exist, it's more about weighing a lot of these factors in their heads and the result of that may be that fewer cases get recognised.