Because the liability is never with the customer in the first place. It's fine that you're worried, but, in reality, you don't really have to be; card gets stol

Because the liability is never with the customer in the first place. It's fine that you're worried, but, in reality, you don't really have to be; card gets stolen? See a charge you don't recognize? Not your problem! Call your bank, get your money back. The one who has to fear fraudsters is not the card owner; it's the merchant! Without 3D Secure, they have to pay the money back! + an extra fine, for good measure. And, of course, the product / service is already gone.