Why design before code when an AI agent builds the screen in minutes: sketch, mockup and prototype

A shopkeeper plans his shop at the counter before opening: a pencil sketch of the shop's home page asking what a shopper must be able to do, four colour swatches, a cardboard model of the shopfront, and a closed laptop set aside for last.

Why design before code when an AI agent builds a screen in minutes: a design system, a sketch, a mockup and a prototype for an ecommerce platform.