The problem is that the individual coins aren't tagged. It's not like cash where you could conceivably note the serial numbers of all the bills that were stolen

The problem is that the individual coins aren't tagged. It's not like cash where you could conceivably note the serial numbers of all the bills that were stolen. You can track the theft and see where the coins got sent, then see where those wallets sent additional coins, etc., but they'll soon get mixed up with legitimate coins (especially if the thief uses a mixer) and you won't be able to know which is which. It would be possible to fork Bitcoin to reject the transactions used in the theft, thereby restoring the funds. This is what Ethereum did for the DAO hack. But so far, the Bitcoin community hasn't shown a willingness to do this.