It's what I said above, it's a lot of work for a new game to create/maintain its own collectibles marketplace that people can trust, and even a well-established game like Counterstrike doesn't properly support third-party trades. Ethereum provides all that out of the box with NFTs.
There's also the issue that Valve controls all the assets, but that's mostly a moot point because they control the game anyway. I guess someone could honor NFT skins in a separate game if they really wanted, but that's getting theoretical.
There's a reason you can't bring your Fortnite skin into a Lord of the Rings game, and it has very little to do with "central companies" and "APIs"