WHY someone has a shit diet is a very important question. Much more important than the copper content of their blood/various tissues.
From what I have gathered in my experience, and putting myself as an example, it's because people have no idea that eating healthy and doing own meals takes less time when given some practice and guidelines than eating unhealthy. Before I have started making all of my meals I was one of those "I'll just buy a frozen pizza" types.
Then I have discovered that by the time I'm waiting for oven to heat up I can make myself a god-damned spinach + veggies pot pie for next three days! For almost the same amount of money that I would spend for one frozen pizza. Maybe I'm just blessed by shitty oven in the dorm's kitchen, but it was a shocking discovery that largely made me pursue various cooking techniques and recipes. Right now I can make myself food for a week in advance for, comparatively, pennies. So it saves time, money and turned out to be much more healthy.
Perhaps actual cooking lessons in schools that focus on making fast day-to-day meals could be more beneficial than some of the alternative public campaigns. I never had any weight to lose (if anything I'm a bit underweight), but I do feel better ever since I have stopped eating crap.