All of the people that use the blockchain as the source of truth and authority enforce it. The question you're asking is literally the same as "what gives bitco

All of the people that use the blockchain as the source of truth and authority enforce it. The question you're asking is literally the same as "what gives bitcoin value". The answer is everyone who believes it has value. Why is TSL/SSL an agreed upon encryption scheme? Because people agreed to use it and you're excluded from access if you don't. When twitter integrates NFTs and requires you to sign a proof of ownership to set an NFT as a profile picture (with a verification check mark), this effectively signals that twitter is enforcing the NFT. When facebook does the same, it signals that Facebook is enforcing the NFT. When Spotify integrates NFT licensing, they will be enforcing it.