We do, actually. It isn't that there's more autistic people, just that the definition has changed so much that it includes way more people. I've often seen people recieve the diagnoses, either for themselves or their kid, then begin explaining away their behavior as if it's entirely caused by autism.
Think of it this way, in the 90s, over 1 million people became autistic overnight due to a redefinition. That doesn't mean 1 mil people suddenly got it, they've always had it, they just are incredibly high functioning.