Maybe this is true if you’re only considering white people. Brown people can spend a lot of time outdoors and still be deficient, especially if their ancestry is from much a much sunnier region or lifestyle than the one they’re currently living in.

Indeed: “Vitamin D deficiency in western dwelling South Asian populations: an unrecognised epidemic” … “27–60% of individuals, depending on season”

https://pmc.ncbi.nlm.nih.gov/articles/PMC7663314/?utm_source...

It has never been established that darker people require the same amount of D as lighter. The supplement industry plays on these fearmongering to boost sales.