given the fact that not even Paizo, arguably the most overpriced major RPG studio, charges what you claim is necessarily to be economically valid.
I think the operative word is paid "fairly". If you think writing RPGs should fairly be a sustainable or comfortable full time job, then the price per unit sold can be on the high side. At 15 cents per word (an average rate industry leaders like Paizo pay their writers in a 2023 thread on this board, don't quote me on the number), you'd have to have 400,000 words published each year to get the mean UK salary. That's writing and getting accepted, an Exploring Eberron-sized book every six months, all by yourself. That's possible (Balzac wrote over 600,000 words per year, but then he worked 15 hours a day and died at 51...), but that's certainly above the output of most authors on the long run. If they can sell 200,000 words each year, they are below the poverty line, and the industry, for them, is paying half of what it should for their wage to be "fair".
But one can also think that getting 12,000 USD from writing a 80,000 words adventure module every 3 years as a side hobby is fair. It pays for nice holidays every year, and it's just writing fantasy, which is cool and not a chore. Fair is a very misleading or context-dependent word.
Edit: Therefore it might very well be the case that the whole industry is paying writers "unfairly", an assessment that may be phrased for many industries. The best football player in the world earns 400+ millions a year, the best fencer in the world earns 0.01% of that. One could both say it isn't fair, and yet have fencing as a viable industry.