Looking for a healthy fast food restaurant, is like looking for a healthy junk food, which doesn't make sense. When a person hears the word "Fast food ", it tells that person somethings, which are snacks, burgers, pizzas, sausages, or others, but there's no way all these foods can be healthy. If a person is hungry, and he decides to go to a fast food restaurant, his expectations will be seeing snacks, junky foods, beverages, that's why the sellers continue to sell these foods to customers because it's what they want. Although few restaurants sell healthy foods, it's rare, and sometimes it can be unprofitable because many buyers don't eat healthy foods, but they prefer junky foods like burgers. The word "Fast ", simply means something that's quick, so a fast food restaurant is expected to serve food quickly, the fastest food to prepare are junky foods, which are unhealthy for consumption.
Since there are preservatives in some foods, sellers prefer to sell them to customers, as it saves them the time of preparing a new one, and it's also quicker to serve. The main reason why restaurants don't sell healthy food is that it's difficult to keep it consistent, consider the preparations, you have to always buy fresh ingredients, there's less processing, etc. In simple words, the healthy foods are more expensive than junks, as fresh ingredients are difficult to get, compared to unhealthy ones like sugar, flour, butter, and more, which are easily accessible or available. Don't forget that fast food restaurants sell unhealthy foods for cheaper prices compared to healthy meals, for example, a burger can cost 0, 90 dollars, but an apple can't be less than 1 dollar. Most buyers come to restaurants to get quick meals that are affordable and satisfying, so if a burger or sandwich cost 5 dollars, while a healthy meal of the same size cost 10 dollars, which would you buy? Most buyers would prefer a burger or sandwich.
In the process of making the meal healthier, it's only adding to its price and delaying the preparation time, so that's why it's difficult to see a fast food restaurant selling healthy meals.
Finally, if a person wants to open a healthy restaurant where healthy meals will be served, then he should be ready to make sacrifices and manage the costs as possible as he can. People like eating healthy, it's just that it's expensive.