Yeah I understand. However, design and art are inter-related. My point is in my humble opinion, design and function is very easy and does not require much talent, while art such as the Mona Lisa requires a lot of talent.
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.
The only thing a designer has to offer is speed. It's like someone who works at a fast food restaurant flipping burgers. You flip a lot of burgers... of course you get fast. You design a lot of simple templates... of course you get fast.