As maxericson alludes to below, it is not about money, but time. McDs is not really that cheap, even when buying a cheap combo or something from the dollar menu. For a single person you're looking at $5-$7.

Meanwhile, my local grocery store has 2lbs of chicken thighs for $5. You can add some potatoes and onions for another couple bucks. So for roughly the same price a single person could get 2-4 meals. The difference is that for the second meal the person needs time to prepare.