We don't need to hypothesize about our bodies not getting the natural opportunities to manufacture it. We know we don't. Plenty of people live in places where the sunlight is insufficient for significant portions of the year to generate any vitamin D. Many others live in places where they may nominally be getting enough ultraviolet to make enough vitamin D, but have too much melanin in their skin for the ultraviolet conditions where they live.
This isn't all about "oooh sooo much indoors too much screen time stop sitting so much lol lol lol". You can go outside all day in the middle of winter at the 45 degree latitude, where a lot of people live, and you will generate zero vitamin D, no matter what you do, because it isn't the visible spectrum you need. You need something that isn't in the winter sunlight at all. AIUI, it's technically not the same part of the UV spectrum that causes sun burns, but you're at least on the right track if you think of it as if you can't burn (modulo melanin), you can't generate vitamin D at all.
This isn't all about "oooh sooo much indoors too much screen time stop sitting so much lol lol lol". You can go outside all day in the middle of winter at the 45 degree latitude, where a lot of people live, and you will generate zero vitamin D, no matter what you do, because it isn't the visible spectrum you need. You need something that isn't in the winter sunlight at all. AIUI, it's technically not the same part of the UV spectrum that causes sun burns, but you're at least on the right track if you think of it as if you can't burn (modulo melanin), you can't generate vitamin D at all.