This is actually very true. There are lots of potential treatments that use generic and non-patentable agents, and very few people are willing to look into it. Zinc is the classic example; some people think it maybe reduces the duration of the common cold. The common cold is one of the main causes of missed workdays. If there's a drug that could make you miss 4 days of work a year instead of 5 days of work a year, it would be worth billions. But nobody is funding studies on zinc, because nobody will be able to capture that saved revenue for themselves. (Zicam did try this; by patenting a nasal administration method and then doing controlled studies with that. If the results were good, then they'd say "it doesn't apply to oral zinc, it only applies to our applicator", and they could have their money. As it turned out, it caused people to permanently lose their sense of smell. I think it helped with their colds, though.)

I don't know about that. From where I'm sitting, lots of companies stand to gain if Vitamin D turns out to make a big difference. For example: American Airlines, Hilton, and McDonald's.

could well be true, same can be said of sleep, the few good papers I've read often showed benefits and I don't recall any instances of harm so safe to try, but no money to pay for anything like the volume of research some rare illnesses get.

Sorry you're getting downvoted for having a healthy sense of cynicism.