Why can't a guardian steal my crypto?

Your share of your private key and your device are the most important part of the network. YOU need to be involved to complete a transaction in YOUR network. It's like you are the king and your guardians are your knights.
As it is your network, you have the final say and authority over your crypto.
Guardians can provide you with security and social verification, but they cannot see or steal your crypto. There is no better way to ensure your crypto is safe.