Actually the seed phrase is the basis of the entire wallet and a private key is only associated with one particular address. As long as there is a private key, they only have control over that single address, however, having the seed phrase gives one control over the entire wallet and control over the entire wallet, including all the addresses associated with it. This is why it is the most critical one why the seed phrase needs to be kept very safe and losing it is like losing the whole wallet forever.