The "common sense" about healthy eating in those countries is essentially the same as it has always been in the US, in my lifetime. Where a home-cooked meal is the only healthy option, and that those who consume processed/fast food are ignoring their health in favor of instant gratification (something that has never been seen as a good thing or even ok). The grocery store carries whole grain options like wheat bread (for many decades), butter vs margarine, and half an aisle of vitamins, because such healthy choices are very popular among people who care.
If you ignore common knowledge about food, then just as with ignoring advice about exercise, maintaining your teeth, and many other things, you will probably suffer. Good health was never presumed to be the baseline regardless of what one ate, it always took persistence and impulse control. The question is why people are doing such a poor job at it now.
Judging from the skeletons they left, our hunter-gatherer ancestors were healthy without need to pay much attention to diet beyond doing their best to get enough.
In particular, they had no difficulty getting enough fiber: there was plenty in everything. Our problem is that our whole food-industrial complex is devoted to stripping out and discarding the fiber we need.
If 40% of Americans didn't brush their teeth or bathe, we would have a hygiene problem of a magnitude commensurate with our health problem.
Roughly that percentage of people don't take sufficiently-good care of their teeth, yes. But brushing one's teeth (and bathing) is easier than resisting food cravings.
The value of fiber (apart from keeping some nutrients indigistible and hence less calories for obese people who need to eat less) is still a matter of debate.
If you ignore common knowledge about food, then just as with ignoring advice about exercise, maintaining your teeth, and many other things, you will probably suffer. Good health was never presumed to be the baseline regardless of what one ate, it always took persistence and impulse control. The question is why people are doing such a poor job at it now.