Explain to me why there would need to be a smart contract with an unpredictable source of randomness for this to work. Coinjoins are running well on Bitcoin, mi

Explain to me why there would need to be a smart contract with an unpredictable source of randomness for this to work. Coinjoins are running well on Bitcoin, mind you. The transaction chains are generally constructed off-chain. For Ethereum you can do fancier things with zk constructs, like Aztec and Tornado Cash.