The mcdonalds/wendys/etc nuggets are junk, injection molded meat paste in 4 shapes. You have to go to popeyes or chic fil a to get actual chicken nuggets.
Relatively cheap, it feels like fast food now has more emphasis on convenience/consistency and less on price/value (in my experience in the US midwest).
Enjoyment and beneficial two different concepts that often get mashed together. Excessive amounts of sugar and salt are often added to food to make it "taste" good and become "enjoyable".
Food quality is Europe is often better then the USA. USA is a heavy user of oil by-product to fertilize the planets, which have less nutritious value than non-oil based fertilizers.
Those in poverty that are on food assistance programs can only use the funds for raw goods. This means no pre-made pizza or McDonald.
* Personally, I hate the idea of going to a restaurant that benefits a Wall Street ticker and a millionaire CEO that pays their real hard-working employees a non-living wage.
In the modern age, if you're poor, or just time poor, you can enjoy a tasty meal thanks to cheap food coming out of the modern food industry.
Why would you pay more for a less enjoyable experience when tasty food might be one of the only joys in an otherwise mundane or hard-up existence?
This is exactly why McDonalds is popular. It tastes relatively good, it's comforting, and it's cheap.