Stolen cars are difficult to hide, risky to transport, and must be dismantled by a trained mechanic before they can be sold as spare parts.
Not a lot of people walk around with necklaces worth $10 Million, and the few who do are usually in the company of bodyguards.
Kidnapping for ransom is high risk operation that often ends badly for the kidnapper. Express kidnapping isn't, which is why it's so common in some Latin American countries. A bitcoin owner is the ideal victim for express kidnappings.
That's why wealthy people don't keep kilograms of gold bars inside their home.
I agree that right now, it's a bit premature to worry about violent gangs coming after your bitcoins. However, if bitcoin ever reaches prices of $1000+, the concerns of the OP are valid. Bitcoins are easier to steal than cars or jewellery, even if stored physically.
Simple offline wallets alone won't be secure enough. Additional features, such as multiple signatories and mandatory lock times for withdrawals will be required.
see:
https://en.bitcoin.it/wiki/Contracts