Because the thing you get, from a prompt like that - even with a sota llm like fable - is a Potemkin village.

Knowing what to ask for, for one. Nobody can just whip up a specification for a system that satisfies all of the technical/design/business constraints that will turn out to have been relevant, has good usability for the target users, hits the right performance tradeoffs - all out of thin air. If anyone could, THAT would be priceless.