Why do university canteens generally require meal cards?

University canteens generally need meal cards to prevent outsiders from entering the campus. Nowadays, the meals in university canteens are generally much cheaper than those outside, and they taste good. This is all because of the support of national policies, and only students can enjoy it during their studies. Of course, this kind of welfare can't be given to outsiders casually. Setting up smart cards in schools can effectively avoid this situation. People who don't have a meal card can't eat in the canteen. On the one hand, it maintains a good dining order, on the other hand, it is also for the personal and property safety of students. Strangers can enter and leave the school at will. Isn't that a mess?