Design talent is more their experience of constant manipulation and testing of what works in certain situations. It's sort of like a UFC fighter and their counters - "if my opponent throws a kick I will check it with my own leg so I don't have the impact of the entire power." Experience is significant here because they wouldn't know what to do in the moment when it matters.

A designer is the same way, say you need a page designed, and they've been through this task so many times they know what works and what doesn't. The lack of knowledge and experience in this area only adds to the time it takes to create & finish something.

And so you're usually not paying a designer to throw a colored rectangle, you're paying them for the years they've invested in testing everything else.