A lot of people here are missing the fact that cereal doesn't require any additional cost, time, and/or effort to store and prepare (in a desperate situation you might even have it with water or dry if you can't access milk).
So while rice or potatoes might be a better meal, and the ingredients cheaper to buy (but not when you factor in cost and time of cooking), they may still not be an option for some.
For those who have never really been it - it'd blow your mind how expensive it is to be poor in so many different ways (a feature of capitalism, of course, not a bug).