What is the human equivalent to dog food or cat food?

I find it remarkable that a dog or a cat can live a long healthy life by eating only one thing their whole lives. My cats eat nothing but Purina Indoor Formula dry cat food. Theyve been doing it for years, and they seem healthy and happy. Apparently, this one food has everything they need. Why isnt there something like that for people--an all purpose food? It seems like it would be the perfect thing for college students and other people who live in financial dire straights because you could make it really cheap. Maybe you could even have it in different flavors for the sake of variety.