> But as soon as you go to sites selling steam keys (proper ones, not resellers),

What is a company/individual if not a reseller if they're selling Steam keys? You cannot sell Steam keys without being Steam or the developer itself, and not be called a "reseller". Or what sites are you referring to here, stuff like Humble Bundle where you get Steam keys with the bundles?

Resellers sell something they bought. Or that's the idea. The sites are marketplaces, sometimes having people sell keys from different countries, sometimes stolen credit card keys. There are several game devs saying they'd prefer people pirating over using those sites.

Real stores sell steam keys because they are selling directly from the developers. Steam is actually nice (or preempting monopoly talk, depending on your view) in that it allows that (I think there are limits, but IIRC rather generous)