Why are they not even mentioning it as possibly a good idea to take? (esp. you are known to be deficient)
Because vitamin D is toxic if you take too much. The correct solution to this is get tested and see if you're deficient. If you aren't taking Vitamin D is a bad idea. I would be interested in seeing a study, if the percent of people with COVID and vitamin D deficiency is the same as the percent of everybody with vitamin D deficiency it doesn't mean anything.