How do you intend to store private keys offline or engrave them on a metal plate? It's not possible that's why with seed phrase, it's human readable and can be engrave on any metallic surface and can be hidden on any location you like, nobody will ever have any access to it unless you tell them.
You can do that with private keys, too. You can write them down on a piece of paper or engrave them. Just make sure you don’t make typos, since one wrong character means the loss of your funds.
But that’s not the point here, as you said, why back up private keys when you can just back up the wallet seed!
Backing up private keys was so common on the early days of bitcoin and when paper wallets were still a thing. But with the introducing of hd wallets, it’s no longer necessary and it doesn’t make any sense.