It is VERY easy to find this information now a days. It's just that most people don't want to seek it out, or they do seek it out and after 2 days and not losing 10lbs, they are done with it and think it doesn't work.
It's also pretty common sense as to what is healthy for most people. Most people know that stuff like fast food and junk food is not healthy for you, and that chicken breasts and salads and veggies are healthy for you.
Yes, if you know you need to look for it. If I told you that working out doesn't burn off fat, you'd probably think I was crazy, because you were told in school that burning calories actually gets rid of calories from your fat stores (spoiler, it doesn't always).
Yeah, some of it is common sense. Some of it isn't. Fruit's pretty bad for you if you eat too much, because it's made of sugar. We were told growing up to eat like 35 servings of goddamned apples a day though. Getting 70% or whatever of your calories from bread and pasta is also pretty fucking stupid, no matter how artisanal it is. I'll
never blame someone for poor nutritional education because they know exactly what they were taught.