Because they are lazy and tell themselves that it's smarter to eat fast food than to cook

I knew someone who rationalised ordering food to be delivered for every meal as more economical than cooking

He became the Chief Financial Officer for KFC in my country

He's fat, and his relationship fell apart

Because they never stopped being children and things that aren't awfulor familiar scare and horrify them. Their brains are fundamentally broken at a deep level and they need probably years of intensive managed psychological help to want anything other than what amounts in aggregate to death in every way that matters while being desperately afraid to die. McDonalds is like a branded fun nostalgic thing that invokes the dying without being scary