1. planning meals (may be minimal if you can ad-hoc quickly)
2. walking/driving to the grocery store
3. cooking
4. cleaning
It can easily be more expensive for an individual person (or even a couple) to cook than it is to eat out. (I think this makes intuitive sense too, since there are economies of scale and efficiencies with how restaurants are run.) If you love cooking, then you are doing it for fun---thats great! But I think for people who don't like cooking, it can be rational to eat out for most meals.
Many people are not paid at an hourly rate, so this analysis may not make sense for them.
(Also, where I live, there are plenty of healthy and cheap places to eat out at.)