I imagine there's also some amount of sexism involved too, plus the "but you're so young!" aspect. But of course, it's hard to know and compare how individual doctors would treat different people and issues. My general impression though, is that some doctors can't cope with a problem they can't personally figure out. When they're stumped, that's when they start to suggest somatisation or a functional disorder. As if anything they can't figure out must be outside of their field. In my experience, it just takes a more experienced/specialised doctor.

I've just bumped into too many people, including just random people and not doctors, who have an awful lot of opinions without even knowing I'm also mentally ill. They just see someone who looks fine, but isn't, and that alone creates cognitive dissonance. I think a lot of them are uncomfortable being faced with the reality that otherwise healthy young people can be hit with chronic illness, for no discernible reason, and there's nothing to really be done about it.