The usual answer is that representing those assets as NFTs will somehow make them portable to other games.

This is, of course, a pipe dream. First of all, there's no business reason for any game to implement this feature -- they'd be undercutting their own sales of cosmetic items by honoring sales of items by their competitors. Second, actually porting assets between games is nontrivial; even if two games are built on the same engine, there are significant barriers to transferring assets between them (like differences in physical scale, art style, or game mechanics).