How can they possibly break even?
It may be hard to grasp for common people but buffets know how to minimize things to pay for. Sure, you may eat a whole lot but it does not mean everyone does. People often come in groups: family, friends, etc. In those groups, not everyone eats like a monster.
In addition, buffets do not need a large amount of staff to govern the restaurant. Some may prompt you to cook your own food. Moreover, buffets use the cheapest ingredients. Therefore, with fewer people to pay wages and cheap ingredients, you already give them much profit.