As matter of fact I was sent to various therapist from childhood to middle age. Many decades. Some were better than others, but even the best I went to never mentioned the word "autism" in connection with me. Part of the problem was though autism was recognized as a thing about the time I started going to shrinks in the Mid Sixties, it was long before 1994 when autism was expanded to being a "spectrum" that included folks who were not as obviously autistic (ie "high funcitioning"), like myself.
The last one I went to was in the early 2000's. My mom sister, and I, suggested to her that I might have aspergers, and she replied that she had never before "had even heard of aspergers" so she read up on it that night. Then later declared that, yes, I have Asperger. And this was atleast ten years after 1994 when aspergers was recognized as an official diagnosis in the US. Ten years and she still had never HEARD of it!! !!
It some years later before I went to a real specialist to get the real exhaustive test, and the real diagnosis.
Things may have improved in the last few years, but yes, I can testify that at one time it definitely took "a long time" (almost half of a century) for the mental health profession to suspect the truth- that I was on the autism spectrum.