People are usually supplementing Vitamin D during the time of the year where there's essentially zero UV light outdoors
Okay look, we are still talking about a symptom and not a disorder. Clearly, the subject of TFA is an aberration, being a football player, ostensibly active and exercising and being outdoors on the regular.
But most people diagnosed with deficiency are not doing any of that.
It really doesn't matter if they're making Vitamin D or whether they're being exposed to UV at all. You could be working in a post in Antarctica, and either you're at a desk job where you're sitting all day and not exercising, or you're working in a storeroom and running around outside doing highly physically demanding things and getting proper exercise. The doctor is clearly going to be concerned about the former person, but what are the indicators?
The people with low Vitamin D need to be retested and monitored during bright summer months to see if the deficiency persists. They need to be up-front about their activities and whether they're exercising properly and getting fresh air at all. The point is not the D nor the UV at all, but the D is what the physicians can detect, and the D is what they will prescribe when it goes off the rails, but what people actually, really need is not prescribable, and it doesn't profit anyone anything, and that's the crux of the problem.
But the trouble with supplementing Vitamin D is that you are treating a symptom. A Vitamin D deficiency usually indicates that you need to touch grass, get outdoors, do some exercise. Since the physician and clinic profit by $0 when they recommend those things, they prescribe you a pill instead. Which idea is a better solution?