I'm not sure that's an apples-to-apples comparison.
In both cases, there is one physical good which, when stolen, deprives you off the money. With cash, it's the physical notes. With Bitcoin, it's the private keys in the wallet (or private key to unlock the wallet's private keys). Making backups of the keys can protect against accidental data loss, but not against theft, as it increases attack surface (i.e. number of locations where the same money can be stolen from).
There is still an advantage here favoring Bitcoin, though: if the key is stolen and you know this, you still have a chance to preserve the wallet's holdings: just generate new keys (addresses) and broadcast a transaction of all the wallet's money to those addresses. If you can get the message to the network's nodes faster than the attacker, the money will be "signed away" before they can use it, and such attempts will be rejected as double-spends.
There is no corresponding feature for physical cash.
In both cases, there is one physical good which, when stolen, deprives you off the money. With cash, it's the physical notes. With Bitcoin, it's the private keys in the wallet (or private key to unlock the wallet's private keys). Making backups of the keys can protect against accidental data loss, but not against theft, as it increases attack surface (i.e. number of locations where the same money can be stolen from).
There is still an advantage here favoring Bitcoin, though: if the key is stolen and you know this, you still have a chance to preserve the wallet's holdings: just generate new keys (addresses) and broadcast a transaction of all the wallet's money to those addresses. If you can get the message to the network's nodes faster than the attacker, the money will be "signed away" before they can use it, and such attempts will be rejected as double-spends.
There is no corresponding feature for physical cash.