As far as I can tell, the definition of autism coincides with the desire of healthcare to address it without having to carve out 100+ variations of behavioral outcomes to get insurance to pay for it.

We know autism affects all sorts of long term outcomes, but if you tried to split it into actual diagnoses, you end up with insurance companies dividing and conquering approvals.

So instead of having several definitions, we put them all behind autism because that has already received appreopiate laws that establish requirements to treat both at school and in healthcare settings.

So basically, once it breached the "we need to address this", rather than every new diagnosis having to struggle to say "look, this problem effects society", it just grows offshoots and spectrum status.

Because it's definitely not a physically identifiable disability. It's all behavioral and that will always have more coincidences.