It’s your kitchen, your money, your headache. Yet you won’t tell your plumber how to fix your kitchen. You may try, but if he thought it won’t work, he won’t do it your way. If you insist, he won’t do the job. He’ll tell you, in no uncertain terms, to go to someone else. Chances are, you’ll back off. He know best.
Yet you’ll rewrite your agency’s copy, and redo their art. Chances are they don’t know a thing. Then why did you hire them in the first place?
Yet you pay them a lot more than you pay your plumber.