It’s not always the complexity and other points you raised: it’s also the sales that the copy will generate.
Outlandish Fiverr scenario: Joe comes to Fiverr to get an article about his thing and pays someone $5 for the thing. The thing goes viral, and Joe becomes a millionaire off it. The seller has $%.
This isn’t as outlandish as I am presenting it, and a lot of talented people (on and off Fiverr) do have this happen to them, although maybe not as drastically as I’ve presented it. All that said, people writing at that level general get options or something in the company (e.g. royalties–for every product sold as a result of my writing, I get 20 cents), but the Fiverr model doesn’t have that built into it–and in any case, most people dwelling on freelance websites aren’t into that, so IDK.
My pricing is mainly based on keeping my workload manageable. I spot a good business idea there for someone with deep enough pockets to break the market duopoly though…