What often happens is, parents are resistant to the idea that their child may be autistic if they managed to get through childhood OK and had a few friends.
But when they get questioned in detail or have to fill out a questionnaire about behaviours you might have displayed, they end up ticking a lot of the boxes for autistic traits - they just hadn't understood that was what they were. Parents often know very little about autism.
The diagnostic process is not solely based on parental reporting, either, you will be tested to check for current behaviours/traits.
Having said that, it's true, if you are on the milder end of the spectrum you might not get a diagnosis.
If you're having difficulties which you believe are from autism and need treatment, though, it's probably worth going through the process, to avoid wasting time with therapies which aren't appropriate.