Consider this, I own a gas-tank filling service. Twice a week while your car is parked in the lot at work I come by with a small tanker full of gasoline, find your car, top off the fuel tank and charge you whatever the going rate is at the corner station. I can do this because being unattached to any major refineries I can pick the cheapest gas and not tied to regional disturbances. You don't spend a lot of money on my service but do get monthly bills and confirmation text messages sent to your phone after each fill-up letting you know the gallons pumped and cost.
One week I send another note saying that you are invited to a Saturday morning social. There will be a small musical offering - your imaginative choice - and refreshments provided. You view it as a kind gesture and considering the morning is free on your calendar, you attend.
It's a nice gathering of people, maybe not enough chairs but no one seems especially uncomfortable. The music is what you expected, not extraordinary or underwhelming. When it comes time for refreshments I bring out saltine crackers, American cheese slices, and root beer.
And you Love it!
Well, hypothetically when I write it you do - but really you don't. Cheap crackers, cheese and root beer probably wouldn't crack your top 100 food combinations and had I run this service most people probably would have left hungry, choosing instead to pay for something they would enjoy rather than something free but distasteful.
This is most children though. My kids and many others see fast food as this tasty treat where chicken should be oozed into its final irregular form. Why? My best guess - my best unscientific guess - is that fast food is an excellent source of nutrition when you had to walk everywhere and the food you ate was the food you caught.
As parents we get into this a layer deeper because it's an easy way to please kids and we build a positive association with it. Going to McDonalds becomes a treat and our children internalize that, not only is the food mighty tasty to their ancestral dietary leanings but mom and dad like when I eat here too.