In my 20 years in software, I have worked for over 15 years in the specific discipline of technical writing. I've only worked in the software industry, and there other industries where technical writers are employed, so my view is not complete. I consider myself a reasonably astute observer of the technical writing craft. I've met some people on occasion who were exceptional, but that's no surprise. I've also met some people who were bad, and that too is no surprise. The surprise is that those unskilled people have not gone away. For the most part, they continue to be employed as technical writers.
Why? I have no idea.
This is what I do know. Hiring managers generally fall into two camps. The first group hires competent technical people and tries to teach them how to write. In other words, hire a geek and fix bad writing with good editing (oh, those poor editors). The second group hires good communicators—that is, writers—with the intent to teach them the technical stuff.
While neither abstraction is pure, I find myself more pleased with the results from the second camp.
In my company, the closer you are to the real, technical horsepower, the more you are perceived as an important, weighty contributor. Given that corporate disposition, it's easy to be swayed to the “hire geeks” camp. In general, I think that geeks are easier to find. Finding a good communicator is more difficult. And because non-geek persons won't actually be contributing to the making of widgets, they are perceived as somewhat unnecessary.
Finding a good communicator who can parse technical information is just rare.
On the other hand, why do the unskilled people continue to surface in the content publishing business?
My experience when working in teams of tech writers, the older ones are usually technicians who feel comfortable with writing and so transitioned their career into TW. I'm reminded of the many stories in the military when men, when professing their facility with a typewriter, were thrust into the roles of reporter or newspaper writer.
The younger TWs I encounter have usually studied it in college at some point, even majoring in it, so their conceptual understanding is okay for parsing information, but their presentation skills lack polish.
But as to your central question: why do unskilled people continue to gain work, I would answer that it's mostly who you know coupled with lethargy. A person you know recommends you for a job in their company. You're not exactly bright, but good enough to pass the initial interview based on the friend's recommendation. Then, once inside, unless you're absurdly insanely incompetent, you exist in the netherworld of neither advancing nor firing for the rest of your life in a shoddily run company.
Thanks for the comments. I don't see the age breakdown quite the same but that could just be my locale is different. I'll have to experiment with thinking about it that way.