Don't underestimate the mental overhead having to cook a meal can cost. Especially if you're already stressed out (and if you're poor you're likely stressed out) and strapped for time.

Nobody is a perfectly rational economical agent, like most comments in this subthread seem to suggest. Stuff sucks, work was hard today, you're tired, and you haven't even started thinking about dinner yet. Fuck. Let's just do hamburgers today. Can't afford them but let's worry about that tomorrow.