There are some insidious factors at play here, though.
The cheapest ingredients are generally worse for you. An example: when health conscious people moved more and more to milk with fat removed, there was a glut (and there still is) of cream. At one point in bulk it was about a penny a litre. As any cook will tell you, add cream to pretty much any food and it will taste better. So, what did food companies do? They added it to low-end foods. Better taste, cheaper ingredients. Thus, the long-term effect of people drinking 'healthier' milk was to move fat intake from people who care about their health into the diets of people who eat pre-prepared food (predominantly worse-off and less educated people). These people aren't eating more or feeling fuller, they're just bunging down meals that food companies have made worse for them because it's profitable. Everyone in this scenario is behaving rationally yet weight is going onto the waistlines of the time and/or money poor.
And it doesn't take a huge amount of extra calories to pile on weight. There are about 3,600 calories in a pound of fat. Eat an extra 50 calories a day (about two teaspoons of cream) and you'd put on 5lb a year, a stone every three years, six stone between age 20 and 40. So, by eating pre-prepared food daily that has had a tiny bit of cream added you could bang on six stone without ever feeling fuller or eating more food.
There's also huge evidence that willpower doesn't actually work when it comes to food. There are other factors at work. Take the bacteria from obese mouse guts and put them into normal mice and the mice become obese. I'm not saying this or cream is the root cause, just that it's not as simple as it sounds.
I suspect there'll be advances in medical knowledge in this area and that solutions will be found but I'm not sure that it's all just a lack of willpower once you look deeper.
Vin