Great tech writers have the skills needed to actually do software development or project management, and often end up either moving into one of those or going to more creative writing endeavors.

This has been my experience as well. The best technical writers often know much about the domain they work in, and can do a fair portion of the work themselves (in software engineering, the best technical writers are the ones coding their own writing tooling)

In fact, many fields actually require their technical writers to have some amount of education in the field (e.g. biology BAs minimum or such for medical technical writing).

Unfortunately, this is a braid field in which other technical writers are performing really basic tasks, like writing straightforward gui instructions. I think this wide range gives the market an excuse to undervalue the discipline, while many writers doing important conceptual writing work are some of the most valuable participants in the arena. After all, nearly all of human epistemic and technological development has been conveyed through the medium of text. What are mathematicians if not amazing technical communicators?

I guess you need to hire engineers who also enjoy writing and are good at it.