You are wrong, Metamask save all the information encrypted on the computer, so if somebody steal the PC, he need to break the encryption.
Alright, that rules out the thief scenario. But whenever the wallet is used, the user inputs some password that decrypts the seed and allows the software to access private keys for signing.
This means that if they have a virus dormant waiting just for that event to happen, it can access the seed. In conclusion, they still don't need an offline backup if using a software wallet, at least not to gain any extra protection from hackers.
But if the user leave the seed on a paper or on plate without be encrypted, the thief you have access to the cryptocurrencies.
So if the user use CryptoPlatz Vault, to store the seed, thief will not have access to the seed recovery phrase.
Or the user could have an actual vault to store valuables and seeds. Or they could have encrypted microSD backups laying around, of their hardware wallet.