I see my debit card and it is limited to 9999 +999 +99 combinations depending on how I use the card. But if the code was alphanumeric as well as had access to special characters the security barrier would be much more insurmountable for the casual cracker. They why don't the banks or whoever is in charge of debit card technology do that ?
Because a gazillion pin-pads all over the world would need to be replaced with full keyboards.
Also, they would be unusable in Arabic, Chinese, Japanese, Korean, Greek, etc.
Most Asian countries have learned to use 0-9 on a keypad, but imagine the issues with a full keyboard.