There's no good evidence that ADHD is not the result of psychological conditioning. For that matter, even if you measured abnormal neurotransmitter activity or brain scans in suspected ADHD patients before diagnosing them, if you looked at animal psychology studies where pathological behavior has been induced by conditioning, I'd be surprised if there weren't abnormal brain scans and neurotransmitter activity there, too. The neurochemical/neurophysical measurements that might provide "evidence" of a non-psychological etiology are invalid if those physical aspects of the brain are themselves affected/modulated by neurological conditioning/development.
If your solution to any problematic "neurobiological" symptom that responds well to a certain drug is to prescribe that drug, because if it responds to a drug it must be a chemical/genetic problem, then everyone will be on a drug eventually. Probably several.