Because just like almost all vitamins, more doesn't mean better, you only need to make sure you get a satisfactory level. If you eat fish regularly, if you drin

Because just like almost all vitamins, more doesn't mean better, you only need to make sure you get a satisfactory level. If you eat fish regularly, if you drink a lot of milk, if you eat a lot of fortified foods, you already have enough vitamin D and more won't help you in any way. If however you don't eat a lot of fish or dairy, and specifically avoid commercial fortified foods (like anyone on a 'natural' healthy diet prepared from scratch, doubly so for vegetarians), and live a modern life indoors without tons of sun exposure, you may have a deficiency and supplements could help. It is far cheaper however to go to a doctor and get tested for vitamin deficiencies, and correct those specifically, rather than spend the rest of your life over consuming tons of vitamins you got sold on that aren't doing anything.