Because it's too easy to claim fraud and have your CC provider initialise a charge-back.
I don't know why you can't go to an ATM or a 7-Eleven and use your CC with your PIN to make a purchase. The CC company will know it's probably you because you entered your PIN and the merchant doesn't risk a charge-back.
First, you can do this with any CC purchase, and I don't see why bitcoin merchants need any special protection against it. Why would a bitcoin merchant be more likely to get a chargeback than any other merchant? Second, this would definitely be credit card fraud, and easily detectable fraud at that; it's highly likely that anyone who attempted this would wind up in jail.
http://www.cl.cam.ac.uk/~sjm217/papers/oakland10chipbroken.p...
The thrust of that paper is that using special equipment it's possible to use a genuine stolen card to do a non-PIN transaction, and that banks will often claim that PIN was used in a card-present transaction in order to avoid refunding money.
In the UK at least, and with a credit card, the bank is legally bound to return the money pending investigation, regardless of the circumstance.
(I agree this doesn't help with the BTC situation because chargeback is still a possibility)