I think my comparison was too absurd/convoluted, because you're still making the same fundamental categorical mistake of asking "Why can't [X] be treated as [Y]?", as if "food" and "drugs" are just synonyms, and not separate categories with different rules and preconditions. But in any case, in a universe where "steak and eggs" were classified as a drug, and have also been commonly sold over-the-counter (i.e. at your local diner, without needing a doctor prescription), then nothing would prevent your doctor from prescribing "steak and eggs" for Alzheimers. And in our current universe, nothing stops your doctor from prescribing Viagra or Tylenol to treat Alzheimer's, even though none of those drugs have been approved by the FDA for that specific use. Doctors have a lot of freedom to use their judgment in prescribing off-label uses. So it's not a question of "Why can't doctors prescribe steak and eggs/Viagra/Tylenol" for my Alzheimer's — it's "Why won't...?", which I think the answer is mostly self-evident.

So why can't Aduhelm be treated like an OTC herbal supplement? Well for starters, because Biogen didn't research and develop it with the intention of marketing it as a herbal supplement. So I think the answers to your question are very much related to the answer to "Why doesn't Biogen release Aduhelm as a herbal supplement?"