You used to be able to easily find parallel-port flatbed scanners for $50 or less, and they eventually shifted to USB. I suspect it was often used to "fluff out" a PC bundle-- "Buy a $1200 AMD-K6 machine and get a scanner and terrible inkjet printer free!" They were probably more than ample for the "scan in this document we need a copy of" use cases-- a few hundred DPI.
While I still see there are a handful of sub-$100 scanners on the market, it's hardly the variety you used to find. It's like a "compliance" product-- being sold because they need to to fill some obligation rather than a vital market full of advancement.
Also note that it's a lot less easy to do the "recycle quality vintage hardware" thing for scanners than printers. Many of them are stranded with ancient drivers and often no support past WinXP or 7. (Yes, there's things like VueScan for ongoing support, but paying for that impacts the overall math).
I ended up finding a scanner I sort of liked used that did have drivers (Epson V39, I wanted something USB powered because I don't have room for any more power bricks) but it involved way too much Googling in the electronics aisle of secondhand shops.
I assume that this is because the market has basically assumed anyone not buying commercial-grade stuff will buy an all-in-one reliability nightmare that won't scan without yellow ink. This sort of thing didn't occur in other markets: They sold low-end VCRs and DVD players built into crappy TVs, but they also still sold standalone ones.