I understand that a smart contract is just a fancy term for code that just does something with accounts in an immutable blockchain. A good example is, if you wanted to send your friend 1 coin a week for 100 weeks. Why can't this code just be stored on a blockchain and have a 3rd party service or another blockchain system read this code to execute it? I understand that this code will have to contain the private key of the sender's account, but I'm assuming this is solved with existing smart contract systems? But either way, why can't the storage of smart contract code be put somewhere else and have another blockchain execute it?