Many people probably do have the same brain wiring, or maybe it's a low level of background dopamine, or whatever it turns out to be once science figures it out.

The reason most of the worlds population isn't diagnosed[0] is that second D: Disorder.

No disorder, no problem, no diagnosis.

[0] outside of lack of access to mental health resources, not knowing what they're looking for so never getting referred to a place to diagnose it, not believing in treating mental health issues, self-medicating, avoiding stigma, etc