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 common in the early days of Bitcoin when paper wallets were still a thing. But with the introduction of HD wallets, it’s no longer necessary and doesn’t really make any sense.
There are many methods for making wallet backups and it's recommended to have multiple wallet backups and different backup methods too. Backups need to be tested for wallet recovery to make sure that they are accurate and usable backups before you fund those wallets with your bitcoins.
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