No "fast food restaurant" is known for being healthy.
People tend to single out McDonald's because it's #1. Trust me Burger King, Kentucky Fried Chicken, and Wendy's are just as bad.
We go to McDonald's mainly for price and convenience. Very few people would claim they make the "best hamburger in the world" and yet they've sold over 9 billion of them!
Fast foods, donuts, ice cream, and other assorted foods are fine if they're eaten as (occasional treats). Whenever someone makes them their (primary) diet they are most likely not getting the recommended nutrients, vitamins, and minerals required to lead a healthy life. One needs assorted fruits and vegetables.
A lot of frequent fast food eaters do not go to the gym or engage in heavy physical activity. Anyone with a sedentary lifestyle who consumes large quantities of fat, carbs, sugar, and sodium is not likely to be in great physical shape.