To be clear, ADHD, despite having "disorder" in the name, is actually a syndrome: a complex of symptoms that, when recognized together, indicate that a certain set of interventional treatments will likely be applicable.
Diagnosing someone with a syndrome does not indicate any knowledge is available on the cause (etiology) of the symptoms. Many different things can cause the same set of symptoms. But if a certain treatment ameliorates anything qualifying as that syndrome, regardless of the upstream cause, then the diagnosis of the syndrome (and so the existence of the syndrome as a concept) is useful, even if it's not informative.
The DSM actually covers two very different categories of what we might call "mental" illnesses: neurocognitive illnesses, and neuroendocrine (or neurohormonal) illnesses.
Neurocognitive illnesses — structural problems with the brain or its cells (think Parkinson's Disease, or ALS, or Lewy Body dementia) — are usually traceable to specific etiologies, as each one usually has either a very unique presentation of signs and symptoms, or has unique markers that can be assayed/biopsied for.
Neuroendocrine illnesses, on the other hand, are almost always syndromes. Many different upstream problems (genetic, toxic, nutritive, auto-immune, etc) can potentially cause the same small menagerie of messenger-chemicals to get out of whack, and due to this, many different upstream problems end up looking like the same few "templates" of symptoms. If you can put the particular out-of-whack messenger-chemicals back into whack with drugs that do that, then you've fixed the symptoms — which doesn't fix the upstream problem (if it even can be fixed), but does fully compensate downstream for the upstream problem.