mcdonalds became famous before any health was any concern, and when it wasnt an issue. they were known for good food, and cheap food, with good taste. if they make everything healthy it will taste crap, lose customers, and lose the legacy of good food cheap.
they try making better alternatives with stuff like the salads, snack wraps (very very good imo), parfait's ect.. but wont change the other stuff they are known for to be healthier because it will lose its well known flavor.