Lot of the low fat/high carb stuff came put of the science of the 60s.
It was thought that because, gram for gram, fat has more calories than carbs, fat = bad; carbs = good. Without any scientific basis, we were told eating fat makes you fat. It made sense.
But not just science. Marketing too.
The whole idea of 'snacking' being good for you. Which means insulin is released every couple of hours.
Introduction of convenience foods that your body doesn't know how to process because it isn't real food.
The hormones that are released when you are full aren't triggered in the are way when you eat junk food, which is why you can eat an entire large pizza to yourself but would be fuller much sooner if you ate proper food.
Being told breakfast is the most important meal of the day when, anecdotally, a lot of people find they eat less if they miss breakfast. Not just because they've missed that meal but because they feel less hungry. And who decided this? Mr Kellog.
If we constantly eat through snacking, our bodies don't have time to burn the energy it has already stored.
I agree with you that the way we used to eat is so far removed from the way many people eat now. And not for the better.