Forgetting Student ID card

Why do the card takers get so upset with students who forget their ID cards and have to give their student ID number instead?  It really shocked me to see those lovely women, who are normally so nice, become extremely angry and annoyed at students who, by a honest mistake, forgot their ID cards.  I understand that not having an ID card can't be an everyday occurrence, but when you have a lot on your mind, like I often do, forgetting your card is bound to happen :)