When I browse through Pubmed and read studies (a/b-tests) of Vitamin D intake and respiratory tract infections (including Covid), many large studies found a positive effect.
https://clinicaltrials.gov/study/NCT05633472
https://clinicaltrials.gov/study/NCT05356936
How do you mean "preventative in nature"? Are you aware of any studies which a/b tested Vitamin D on active respiratory tract infections which showed that the Vitamin D has no effect in this situation?
I’m just calling out the fact that your complaint is not rooted in reality.
The studies you refer to are observational tracking people who supplement vitamin D or live in places where they get it naturally more. To say vitamin D is the cure to most things or even long covid is shortsighted and frankly irresponsible. Linus Pauling did that with vitamin C and tainted public opinion on vitamins.
A huge amount of study does not lead to patentable medication, but is either performed by research groups, or is performed as part of R&D of another products and then published in papers.
There is a difference between something that could never be patented (Vitamins), and the possibility of something working becoming patented (Prozac for example).
The first does not get the funding but from benefactors, as in the case of the study I collaborated with.