Do you know your credit card number-

[Pyrrho]

And if not why not?

I do not have my credit card numbers memorized because it would be a waste of effort.  I do not need to know the number of any of my credit cards.  They are conveniently printed on the cards themselves in case I ever need them.  And they are also on the bills, so even if the cards get lost or stolen, I would have the numbers for reporting them (which, however, would not be essential, as they will be very interested in canceling the cards just from my name and address, as they will be stuck with the loss, not me).

I think a better question would be, why bother to memorize them?  Of course, if one memorizes them accidentally due to entering them for online ordering, then that is something quite different from taking the trouble to try to memorize them.