I think the biggest issue is that people assume that writing is a skill that "anyone can do," as opposed to something like math, which is considered a more narrow focus that you need an "expert" for.
This is of course, not the case. Writing is as much a skill as anything else. You can be be an expert in writing like anything else. Like anything else, there is a difference between good and great. And like anything else, if you're great, you should get paid to do it.