In addition to the fact that the very idea of NFTs is a scam, Valve doesn't want to enable money-laundering lest financial regulators drop the hammer on them. Banning any games that involve real-world assets (or anything easily convertible to a real world asset like crypto) is just smart business given current financial regulations.
It also gives them a complete monopoly on their internal marketplace, so that's a nice bonus.
It also gives them a complete monopoly on their internal marketplace, so that's a nice bonus.