The right comparison is:
a) an offline wallet on a safe medium stored in a safe location
b) a physical bank note in a safe location
See the "safe location" part? That's exactly the same for both an offline wallet and a physical bank note. There is, literally, no difference. You can make the safe location a hole dug into your garden or a locker inside a bank-owned vault.
Once you bring in bank _accounts_, you bring in a whole different world:
a) once you "deposit" money on an account, it all starts with you actually making a loan to the bank (with all the risks that entails should the bank go into default!); you cannot do that with just your personal BTC wallet
b) once money is in a bank account, you typically make use of services and offerings to do something with the money - e.g. by using plastic (bank card) to pay; if you want to have the same "service" with BTC, you need to expose your wallet exactly the same way. If you want use your BTC at a point-of-sale, where does it come from? Well, either from your _online_ BTC wallet, or from a crypto chip on some plastic, or from a digital device (e.g. a smartphone) inside which you carry your BTC wallet sort-of offline.
The means how BTC and traditional currencies can be attacked are different - e.g. someone could simply induce strong electro-magnetic field fluctuation to your offline bitcoin wallet. Without a backup, your wallet would be lost. On the other hand, it is possible to even locally back up your wallet, so in the case of, say, an isolated failure of one storage medium you would retain your wallet. Try that with a bank note ("my dog ate my CHF 1000 bank note!" - expensive dog, huh?).
The one fundamental difference on currency "value" between traditional currency and crypto currency is that traditional currency tends to be backed by a "promise" by a central bank / a sovereign that it can be used for storing wealth and used as a transport medium of "value" in trade (fiat money - backing). There is no such player in crypto currency, so there is no such promise (and the crypto currencies are designed such that they do not need such a central player, either).
In closing, I have to reiterate one thing: Structurally, an offline BTC wallet can only be compared with a stack of physical currency notes or currency coins. Attack vectors on both exist, but are different in realization and risk.
_Bank accounts_ for traditional money are something completely different - they have many more contracts, implications, and services attached which go way beyond what is associated with a crypto currency wallet.