Usually they are mediocre chefs. Actually I don't believe that a chef who doesn't like what he's cooking can make it right, and almost every awesome chef I know demand more from the quality of their dishes than their customers.

The product of a chef is the cooking, not the cooked product itself.

I often buy better ingredients than what's found at any restaurant, but I pay to sit at one because they're prepared, cooked and taste better when done by a professional.

Often (not always), you can clealy taste the difference.