“Who made that rule up?”
Nature decided. You should be able to achieve healthy levels through normal eating and outdoor activity. The human race would have died out long ago if that wasn’t the case. Supplements are intended for people who have a disease or some kind of dietary issue that limits nutrient bioavailability.
If you can't see the change, that's on you. Diets have changed, too. and lot of folks died from dietary deficiencies back before electic lights or printing or any of the modern stuff.
No rule there. Just your belief that you know best.
/johnny