The main reason is that I understand the market. It's just not feasible for RPG publishers to pay .25/word because their sales won't justify it. You don't expect small-press publishers to offer a $20,000 advance, but you might very well see it out of Random House.
It's justifiable in other ways. You don't have to be particularly talented to write for the common rates these days. The higher rates usually have some competition, but if you can write complete sentences and your work has a beginning and an end, you can find a buyer for mediocre RPG material. Even at the top end, if you show knowledge of what's already on the market, show moderate development and design skills, and have high school graduate-level writing skills, you can sell consistently.
Only for the top payers do you really need any real proficiency in the medium. For most markets, I can sell first drafts, and they appear more or less as written. Few RPG publishers have (or are) editors with even basic copy editing skills, much less substantive editing. They don't know or appreciate the difference between weak and average writing or average and good writing.
This might sound like I'm denigrating a market I write for, which some would consider foolish. I can justify every observation, though. I once castigated a publisher for offering .01/word. I took a sample he had provided and edited it. I found something like 17 errors in a 300-word sample. The errors ranged from punctuation to design flaws, and they were rampant throughout the article. I tried to explain that THIS is the difference between .01/word writing and .05/word writing. Offer higher rates, get better writing, sell more stuff. Publishers don't appreciate that superior writing--a skill which costs money--equates to better sales.
A lot of this attitude comes from the current environment of PDF/POD publishing allowing any hobbyist to become a publisher. They start with no business plan, no money in the bank, and no real goals other than "sell some kewl stuff." They don't budget for good writing, and by putting out poor quality product, they ensure that they'll never be able to bootstrap themselves up to a position where they can afford to pay for good writing.
Also, many fanboys will write for free or nothing, which really sets the market. A publisher can fill his magazine or book with low-quality material and sell x copies at y cost, where y is practically 0. Paying for top-notch writing would sell x+25%, but cost y+500%, making it non-feasible.
Many "publishers" in this environment are happy to sell 100 copies of a PDF. Yes, that's 100 copies of a PDF that might sell for $5 or $10, and a 25-40% cut goes straight to the hosting website. They might sell a few POD copies through Lulu.com. They might go to a couple of conventions where they set up a table, take a couple of copies with them and sell or give them away.