In England/UK, there is a permanent recommendation to supplement Vitamin D b/c of endemic deficiency, given out by the public health bodies: Public Health Engla

In England/UK, there is a permanent recommendation to supplement Vitamin D b/c of endemic deficiency, given out by the public health bodies: Public Health England (PHE) (https://www.gov.uk/government/news/phe-publishes-new-advice-on-vitamin-d https://www.gov.uk/government/news/phe-publishes-new-advice-...) and NICE (https://www.nice.org.uk/guidance/ph56 https://www.nice.org.uk/guidance/ph56). It is not strictly Covid-19 related, it predates it. It is just not publicized enough, and not acted on sufficiently. Imho you are right. Given the balance of risks/costs of intervention (Vitamin D safe, cheap, prior knowledge population deficient & helps respiratory illness) versus not (pandemic) it is a no-brainer: to increase the uptake (via better publicity and more). As to "why is it not done", I think: the default is "no intervention", and it has not found its champion to push for intervention. As simple as that.