2) Taste. To us adults (and mostly upper-middle class adults to boot) on HN, rice and beans may sound more appealing than cheap mac and cheese with mystery meat. Take a poll of 3-year-olds and you may get a different result.
3) Ease. Staples like rice and beans are only really cheap when you're buying them in dry form, bulk. This means you need to season them and cook them, which can be a long process. (My wife and I recently tried to save on black beans by cooking ourselves rather than buying canned. The result was good, but by the time you chop onions for seasoning, add other ingredients, monitor the beans, drain and cool the beans, and wash the crock pot, cutting board, knife, various spoons and measuring implements, etc., well, it ends up being a lot of work.) If you're a single parent working two jobs to support your kids, 5 minutes of preparation is meaningfully better than even, say, 20 minutes of preparation.