The reason they are so cheap is that EU laws protect the consumer right to resell things they purchase, and EU feels very strongly that this extends to software licenses. Most pre-built computers come with a bundled OEM Windows, which any EU resident can legally sell. Since a lot of computers bought by businesses, schools, hospitals, etc also contain Win 10 Home licenses, which are promptly replaced by whatever OS the site actually uses, and then immediately sold to someone reselling them, there is a healthy marketplace that has pushed license prices very low.
[1] https://en.wikipedia.org/wiki/Windows_10_editions#Comparison...
There's a full summary of the legal situation(in Polish, but I'm sure google translate will manage), with legal documents supporting it(it's actually been going on in early forms since 2000, but it's the ruling from 2012 that has cemented your right as a consumer to sell on software, regardless of whether the manufacturer allows it or not)
https://www.forscope.pl/blog/gwarancja-legalnego-oprogramowa...
It would just mean that enforcing all terms of that license would be impossible.