It's because they're usable for the game. Unlike the NFT arts and other static NFTs, they're good like you're holding a painting and any other memorabilia and you'll only make a profit out of it upon selling it.
But with NFT games, you buy the NFT alone and that's going to be your starter that you'll use to generate income as you play the game. That's why the potential of the NFT games is very big.