I thought exactly what you thought for 50 years. Changed my mind via experience the last 4.
It may be overdiagnosed, but IMHO it is there.
BTW what do you think of autism?
Autism is over-diagnosed, but full-blown autism is at least a real condition. ADHD is bogus. This doesn’t mean that there aren’t kids who exhibit the “symptoms” in question, just that those symptoms are due to parenting, not brain abnormalities.