It used to be the case that they were all keys that aren't technically allowed to be resold per their agreement. Either OEM or business copies and the like.

So really we're back to some form of piracy because Windows is too expensive.

You say that used to be the case; has that changed? How does it work now?

You don’t even need to buy a license. Alternate activation servers exist.

Which is certainly a form of piracy.

I went to Microsoft's store and found the entry for Windows Pro. It's the actual price.

Other comments have pointed out why G2A is also more or less piracy except you pay someone for an illegitimate key instead of using a cracking tool to generate one.