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.
It's true but if you know which addresses have bitcoins, you can simply backup private keys of those addresses if you want. Surely it's more easily and even more safely to backup a wallet mnemonic seed words because it's easy to do such wallet backup and I don't see big reasons for don't do this and why a Bitcoin user must skip mnemonic seed words backup to use specific private key backups.
Even Satoshi Nakamoto recommended that Bitcoin users should never delete their wallets.
Sigh... why delete a wallet instead of moving it aside and keeping the old copy just in case? You should never delete a wallet.