It is because when you are working and studying you are miserable. Your subconcious knows it, your soul knows it etc. So these junk foods are to cope with not being authentic. Humans were not designed to be at a desk looking at small writing all day. They were meant to be out exploring, having sex, fighting etc. When I was studying at a career I hated I had many self destructive tendencies. Now that I am free I dont have many of these tendencies.For example, when I'm only working and studying... I tend to eat more junk foods. When I start to socialize more, have sex, going out and even drinking some gin, I eat better. It's like changing a scape valve for another. Or maybe this is just a addiction mindset I don't know.