A lot of foods that are really bad for you are kind of a pain to make. Potato chips. Fried chicken. Anything deep fried, really. Coffee cake. Chocolate chip cookies aren't hard but aren't exactly make-on-a-whim either and require resting the dough for a good long while if you want them to be really good. Most stuff that's easy to eat within minutes of a craving without having it prepared in advance are whole fruit or salads or something like that. Nuts are probably among the worst ready-to-eat foods but at least they're full of protein and vitamins, and arguably their eat-quickly-ability is well balanced with their nutrition level if you buy them in the shell, which prevents tossing back whole handfuls at once. The truly bad stuff takes planning, unless you buy it already made.

[EDIT] oh man, crackers! A fair amount of effort for a pretty small yield in a home kitchen, even using electric mixers and such. Enough of a pain you'd hesitate to just eat them as-is rather than using them as one part of a larger meal. But a whole box is a couple bucks at the store....