I was diagnosed with severe crohn's disease in the late 90s. Doctors started learning about Vitamin D and the immune system in the 2000s. My only specialist who knew about it in the 90s is now at an elite hospital. It was rare knowledge 20 years ago. Medicine is still pretty much stuck in the last century with nutrition knowledge. It's all about pushing pills for the most part. The synergistic study of different nutrients is still pretty lacking.

For example, most Drs now know about vitamin D. But do they know about A and K and the dosages and which minerals you should probably take with it when someone is extremely D deficient like in a lot of autoimmune patients? Probably not...

Do doctors know most patients with resections or digestive issues who are B12 deficient can absorb methylcobalamin sublingually? I've never met one that does.

I can go on. Most doctors understanding of B12 is still stuck in the 1970s along with most other aspects of nutrition.

The old-school, prevents rickets/400 in milk is enough, is what was taught for decades. The new-school, take a few thousand a day and it prevents winter respiratory infections is quite newer and less spread out.

Personally, I stopped having the vast majority of colds during the winter since starting D a decade ago. One or two colds since, and both were over in a day. Definitely a significant life improvement.

There are several dominant threads pushing back here on a professionally done study about the benefits. So no, the important information is not as widespread as you claim.