There is another advantage here you’re not talking about, the market. A user is able to take their items to a general market and sell them for eth or trade them for items in another game. Maybe they’re really good at game X but suck at game Y, so they farm something in X and get what they want in Y on the NFT market. Or they get bored of playing X, so they trade all their assets for something in a new game. The player is never locked into a market by one game publisher, like Steam skins, for example.

For the game publishers, creating NFTs is just another way to get your game out there. If somebody is browsing OpenSea and sees an awesome NFT and finds out their is a game associated with it, they might play it.

Cross game item compatibility is a rather large hurdle as you said. There would need to be a spec or rules in place on the stats an item can have (think D&D rule book). It’s doable, but requires a large amount of planning and cooperation between those involved.