I see the OP's point. It seems that often autistics are like "as well as autism I have A, B, C, D, E, F, G and H", and I think, "all of those?" I mean, probably some of those disorders overlap so much that it's not really worth being diagnosed with.
I mean I have PDD-NOS, ADHD and anxiety (all diagnosed). I think I have OCD too, but how do I know my OCD tendencies aren't just a combination of all 3? I might as well put OCD down to those instead of getting another diagnosis, then another for something else, then another for something else, then another, then another...
Not invalidating those who do have a number of disorders, but mental disorders/disabilities are more complex than physical disorders/disabilities so it's not something that can be diagnosed via a blood test. It counts from behaviour patterns, development, etc, and can sometimes be difficult to decipher which behaviours come from what, the more diagnoses you have. Autism just steals all traits from all diagnoses, as I'm pretty sure I have heard every human trait possible (even normal NT traits) be "an autism thing".
One time RSD was more associated with ADHD, but nope, now it's part of autism, just like everything else. Lol.