Mother and father are working longer hours with no set office hours, the kids are constantly on the go with athletics, school clubs or after school programs. Nobody has the time to really take the time and prepare a decent meal. Heavily processed food just became a substitute for the time it took to prepare a real meal. Fast food then became a crutch for families on the go even replacing the heavily processed food that replaced the real food people used to prepare.
The end result is kids and parents both eating poorly, heavily stressed at school and at work. They eat poorly, they don't get enough exercise and now they gain weight and other issues that affect their health.
As many people have pointed out in various other threads, in the 50's and 60's people hardly ever ate out, the family dinner was a big deal and most families ate at the same time every day, had set schedules and the kids exercised a lot more (whatever happened to recess??) by playing games, athletics, and using their imaginations.
It's really hard to disentangle plausible causes. But I agree, something must have changed, something that makes this problem correlate with "westernization" and technological advancements of societies.
I think some people just have their internal "hunger" thermostat set wrong- whether it's hormonal or psychological or whatever. Other people have it set more correctly so by and large, "willpower" doesn't come into it.
I don't need willpower to stop myself overconsuming water, but if I was thirsty literally all the time, I'd drink gallons.