And these haven't been used to illegally obtain fiat on millions of occasions?
Of course; so we know that the danger of having keys stolen (particularly from users who are not computer security specialists) is not just a theoretical possibility.
Now compare a trojan that steals bitcoin keys with one that steals bank passwords or credit card data.
The bitcoin trojan can immediately sign and send off a request to transfer all the bitcoins in the compromised address to the thief's address. The theft will be complete and irreversible before the victim has a chance to check his balance. The thief does not have to intervene or even monitor the action, and he can be anywhere in the world. Once the theft is complete, the victim has no recourse: he cannot even prove that the transfer was a theft. A million such thefts can be performed on the same afternoon, or a single address can be emptied of one million bitcoins, and no one will suspect of anything.
A trojan that steals a bank password may try to send the stolen data to the thief, but that is risky since that communication could be traced. Instead, the trojan could perhaps log into the bank by itself and issue a wire transfer to the thief's bank account. The transfer may take hours or days, depending on where the thief is. During that time, or even after it, the client may notice the theft, call the bank and cancel the transfer. If the amount it too large, the bank may get suspicious and call the client. Ditto if ten clients empty their accounts on the same day.
Either way, the thief still has to get the stolen cash out of the bank, within a couple of days at most. That is risky because accounts owners must provide identity, and cash can be traced across multiple bank transfers. The thief cannot open thousands of accounts to receive the stolen money. And so on.
Credit card theft has similar difficulties and risks.
Therefore, it is not surprising that bitcoin thefts are already so common, in spite of the small "market" and the novelty of the concept. Bitcoin theft must be the hottest thing in the black hacking field now. Bitcoin is still wearing diapers, and yet bitcoin theft already amounts to well over 500 million dollars -- perhaps more than all the money that has been invested in bitcoin ventures so far (SMBIT, for instance, has got only 50 million).
As bitcoins become more widely used, we can be sure that the hackers who now steal credit cards will turn their weapons to bitcoins.