The other thing is that in many places, like the US, the supplement market is largely unsupervised so even when the company is says they are selling something that has a therapeutic use it is likely that the product is adulterated or the dosing all wrong.
One recent example were some homeopathic remedies which should have been harmless (homeopathic remedies should contain essentially no active ingredient by design) but where some turned out to contain lethal levels of belladona due to shitty manufacturing processes. This ended up killing quite a few baby as these were marketed as helping teething babies.
Or all the protein powders that turned out to be mostly various carbohydrates when actually tested.
etc etc