Astute analysis, Valve's proximity to the secondary market seems to be the issue here. They also appear to allow you to generate keys for games that can then be sold[0][1]. This provides you with an indirect way of cashing out your earnings.
They don't allow end-users to generate keys for games - that is restricted to the publishers or game creators (in smaller cases).
The keys on those sites are generally sourced from physical copies in cheaper regions (as mentioned in a sibling comment), cheaply sold bundles (Humble Indie Bundle, and other similar sites), and other such sources.
You could possibly make an argument for gift-copies of games[0] being resalable, or games that can be added to third party systems (ME2 gives a CD key that can be added to Origin, for example) though the latter would be limited to one attempt per-game per-account.
[0]https://www.g2a.com/call-of-duty-modern-warfare-2-steam-cd-k... ($7.23 USD vs Steam price of $19.99 USD)
[1]https://www.g2a.com/mass-effect-trilogy-origin-cd-key-global... ($13.66 for all 3 Mass Effect games or $34.99 on Steam)