I think the majority of Americans choose food based on how it tastes, with the relative healthiness of the food being, at most, a distant second. And while I, personally, believe that there is food out there that is much tastier than Mickey D’s, they have spent a lot of money on taste testing, focus groups, and food “science” in order to figure out just the right flavoring agents to spray on their food to make people crave it.
Also, there’s the ad budget. And, how “cheap” it is, as long as you’re only looking at the cost on the menu and not what it does to your body. (With more and more working class and poor people, this is a concern.) There’s also time constraint – it doesn’t always take 35 minutes in line, and even when it does, that’s still less time than it takes to make a real meal with real ingredients on a weeknight.
Another thing to remember: some parents who have better taste and sense also find it much easier to compromise on those than to listen to the kids whine and whine about McD’s, and will sometimes do anything it takes, including eat fatty garbage food, to shut them up.
Cost, convenience, engineered flavors, an ad budget, and being a cultural institution – it’s amazing more people don’t eat there. People, very often, are sheep.