Before I started self-publishing, I freelanced for over 15 years. I was never able to set my own rate. The publisher offered what they could afford to pay, and there was never really any room to negotiate. I worked for some name-brand publishers from about 2000-2015 and never settled for less than 2 cents/word, but was never offered more than 5 cents/word.
It seems like discussions about RPG writer pay tend to take place out of the context of the publishing business and how much (or rather, how little) money RPG publishers actually make. Direct-to-consumer sales (which are now more prevalent than they used to be) helps by allowing publishers to keep more of the cover price, but under the traditional three-tier distribution model, they don’t keep much, and out of that much they have to pay for all publishing and operational costs, not just text.
It’s useful to compare RPG pay rates to what technical and copywriters can make, but only if you consider that those rates are paid out by companies that make things or provide services that people or other companies need, whereas most RPG publishers are hardscrabble operations that struggle for a cut of the market for a purely discretionary product (as are all forms of entertainment). In my experience, these discussions tend to imply that publishers are just cheaping out on the price of labor as if they were running a steel plant or a coal mine in the 1920’s. The real problem is that there is too much product competing for a market of hobbyists that, while it is much bigger than it was 20 years ago, is still relatively modest and discretionary. It’s hard enough for most publishers to make money for themselves, much less pay writers what they ought to be paid in an ideal world.