Because way back in the old days they used it have these machines that would run over the card and clone the card information onto carbon paper.

That's why the number is raised on the card.

But those machines are for point of sale purchases only and should NOT get enough information for the card to be used over the phone/internet. Otherwise you'd have tons of carbon sheets that could be used to defraud people.

Also as the other poster said, harder to photograph using a card capture device attached to an ATM/cashpoint.