Because if two people have a copy of the wallet file, you've got a situation which is essentially impossible with physical currency.

I'd liken it to two physical credit cards with the same number, but that's a poor analogue as the feds can just ring the card company and have the account seized regardless of the physical "pointers" to that data.

Bitcoin, access to the wallet file = access to the funds, at least until there's a spend. I believe that makes it rather unique.