The issue Valve had with this was that they weren’t willing to do the basic vetting needed to ensure a mod’s content is legitimately owned. For a full game, made from scratch in Unity, that’s not necessarily easier to verify, but the bar for entry (and to making something interesting for viewers) is high enough that a developer can be harshly penalized for breaking it. And of course, it has still happened, wherein a little Superman game is found to have been completely stolen from someone who put it out for free.

When you have a big complete game like Skyrim, and one mod only needs to do something silly like put Thomas the Tank Engine’s face onto a greathammer, it’s a low bar for entry for something some people might actually want, as opposed to shitty Unity asset flips. Still, the storefront needs to be sure that THAT developer owns Thomas’s face (they don’t) and that they put the work into applying it (they might have just pulled files from some obscure Nexus Mods entry and hoped no one noticed).

Ownership verification is tough. I seriously doubt they’re actually putting the time in.