Because they use the web everyday.
At my old company, we taught designers contraints about web elements and what could typically be styled and executed. This relationship between front-end dev and web designer was pretty key. I never coded but because of my usage of the web, I knew the constraints.
Those designers ended up designing some of the most used sites in the United States.
When working at an agency, one rarely ever touches code because the client either has a team to deal with it OR there are full-time front-end devs.
But hey, if those designers who multiplied revenue for ecommerce sites and increased viewership for content sites are bad web designers, then I'll take those scrubs anyday.