Because McDonalds is known for serving healthy and nutritious foods?
I think McDonalds has actually been trying to improve their image in recent years. They publish nutrition information on their websites and in their restaurants. At least they tell you what you're actually eating now.
Even their restaurants appear to be looking a lot better these days. Seems to be much less gaudy, eyesore red and yellow plastic now.
I'm sure there's much worse than McDonalds around as well. I had a White Castle burger once in NYC, that was absolutely disgusting. Was rather like that scene from Demolition Man where Stallone is eating a rat-burger.
There's one thing I will give McDonalds though, at least things like BigMacs and Quarterpounders are consistent, whether it's Bristol, London, New York City, Beijing, Hangzhou, Shenzhen or Hong Kong. totally unlike Dairy Queen. What Dairy Queen in Hangzhou serve up, is just horrible.
KFC seem to have some consistency problems as well. While any KFC in Beijing seems to OK and what you would probably expect, KFC in Xilinhot is NOT a good experience for a number of reasons.
I actually quite enjoy eating at McDonalds and Burger King occasionally. Get some good old western junk food down me, makes a change from healthy Chinese and Mongolian food.

Nearest McDonalds to me is a ten hour bus ride away in Beijing, so I don't eat too often there.
Unfortunately that Real Fruit Smoothie offer doesn't seem to be valid in China for some reason....bummer.