Vitamin D is fat soluble and isn't cleared by the body as quickly as other vitamins such as B. Therefore it can be taken in higher doses, but less frequently. This is often done for convenience sake. A 3000-5000IU pill every week is easier to remember than a 500-600IU pill every day.

Vitamin D is fat-soluble and can be stored in body fat. Supplements in the higher range are meant to be taken once a week or once a month. Supplements in the 400-1000IU range are meant to be taken daily.

I don't think they are, though there certainly are supplements in that range and higher.

But supplements don't mostly sell based on either science or government recommendations, but fads which are often unrelated to either.