It's not just that the science of nutrition is still in a state of heavy churn... I'm sure everyone remembers when high-carb, low-fat diets were supposed to be the best thing ever, and we still suffer from the effects of that with tons of "fat-free" products that are full of sugar and unsatisfying.
It's also the governments fault for things like corn subsidies that assure that every processed food product is pumped full of corn syrup, because it's cheaper than air and making things sweeter is the cheapest way to get people to consume more.
The food industry is also working against the health of people because so many of the things that make their products more appealing also make them worse for you.
I cook a lot, so I eat less processed food than the average person probably does, and I know I need to cut down on carbs, but I eat rice with most meals. If I could eliminate some bad habits, I don't think my diet is that bad, but eating healthy is also expensive. For all the talk of childhood hunger in the U.S., obesity is more of a problem for the poor than hunger. Cheap food is often really bad for you, and busy people with more than one job can't afford the time (leave alone the money) to cook decent meals unless they are really dedicated and make an effort to educate themselves.
I cook a lot because it's fun and relaxing, and usually make a bunch of food on the weekend so my family and I don't have to cook during the week. One of my problems is that I get bored easily, especially at work, and I eat when I'm bored.