You do realize Rain Man is a true story.
And?
Rain Man isn't a documentary.
Also it's not a true story but the character who has autism was based off a real life person who had quite strong autism.
The fact is people who have autism were often compared to that particular character (and the savant abilities).
There is a massive contrast between people with autism. I've been with people who have severe autism and I've been with people who have much weaker forms.
Rain Man gave only one view of autism and that's how the public took it.