1) You don't need to speculate what neurobiological means using phonetics, it's a real word with a definition you can just look up if your actual interest is in the facts. Neurobiological actually means related to the nervous system of the body. ALS is also neurobiological, and I'm guessing you wouldn't say it "sounds" psychological.
2)If you'd bothered to look, you'd find there is actually quite a bit of study of D2 and D4 availability correlated with ADHD symptoms. Like virtually all medical diagnoses both psychological and physiological, differential diagnosis is done by combining probabilities with presented symptoms, and not with exhaustive testing. Your requirement of diagnosing ADHD with a brain MRI is the equivalent of requiring endoscopy to diagnose an ulcer.
So what you say is factually incorrect, there's plenty of evidence. That's why the scientific body of many countries including the US has classified it as neurobiological (not psychological) for years, and the bulk of the scientific community believes pharmaceutical treatment is necessary in the same way that a broken arm needs to actually be set instead of telling the patient not to be lazy.
If you want to use scare quotes and insist on unfalsifiable assertions "prove the negative 'not' the result of psychological conditioning" then you are placing your argument in the same category as climate change deniers and anti-vaccers - anti-science, anti-rigor, pro-scare quotes.