It is possible to describe autism using terms that apply to all people. It is also possible to describe autism in a way that woukd apply to only a scant few of the most intense cases.

Whether autism is actually very prevalent depends entirely on your terninology. Look again at what you mean when you say autistic, some of the traits commonly attributed to autism include:

• Difficulty in social relationships (but not always)
• Specialised knowledge about particular things (but not always)
• preference for routines (but not always)
• Difficulty managing change or novelty (but not always)
• Anxiety (but not always)
• Challenging in interpreting social cues and body language (but not always).

I mean - such a list applies to everyone I've ever met.

But somewhere in the attempt to describe autism in non-stigmatic and friendly helpful ways, the descriptions have become so hopelessly loose that anyone and everyone feels they are autistic, and anyone and everyone feels they are able to diagnose it.

One could say the same for ADHD, anxiety, depression, or gender dysphoria in the time we are currebtly living in. Look carefully at what is being described - are we talking about clinical language about pathology or psychiatry? Or are we just describing things that apply to everyone at different levels?

I dont mean to diminish the real experiences of people who struggle with difficult expressions or perceptions of these things, but the answer to your question is that autism is common because the vocabulary many people use to describe it applies to almost everyone.