You're not wrong about that! People for the most part do know that soda and burgers aren't healthy.
But there are a lot of products out there with more grey area than the typical McDonald's menu item.
For example, the Dunkin' "Power Breakfast Sandwich." The marketing sounds healthy without saying the words "healthy:" https://www.dunkindonuts.com/en/menu/sandwiches-and-more/pow...
Notice how this item has almost half your daily value of saturated fat! High cholesterol, high sodium!
A lot of people would see this menu and think "this is a good alternative to McDonald's."
Same deal with Subway. Jared was selling us on weight loss via eating sandwiches, but he himself didn't eat the Sweet Onion Chicken Teryaki – a.k.a. Chicken on Bread with Sugar Sauce. Subway was promoting footlongs (five...five dollar...five dollar footlongs!) specifically, a portion size that is objectively too large for a single meal unless you're skipping a meal or eating with basically no condiments. Everything about Subway is healthy-ish until you get to the end of the line and you have a spread of caloric sauces to choose from (all free) along with a translucent case of cookies, addictive chips, and unlimited-refill soda.
Another example: products labeled "no added sugar" that are largely sugar, such as orange juice. Sure, it's slightly better than added refined sugar, but it's still sugar and it's still calories.
I once dated someone who thought their daily Dunkin' Donuts coffee was a low-calorie menu item. Nope! It's 300+ calories, because it was a latte in a large portion size with sweetener. But it looks really similar to low-calorie items.
Portions are a huge issue, too. A lot of those fast food chains make larger drink and side sizes in the USA compared to the exact same menu items overseas. Why this is, I'm not actually 100% sure – maybe a more competitive fast food market in America where these companies are trying to beat competing restaurants in value?
All of this is to say:
1. We can't assume the average person's nutritional knowledge is very good. It's barely taught in school and the schools themselves are doing basically nothing to promote healthy eating. My public school provided meals like pizza, smiley-face french fries, Little Debbie desserts, and an entire cookie and slushie stand. They actually helped get me addicted to bad food!
2. We can't assume that the general population has any idea what rational portion sizes look like. This has been distorted beyond belief.
3. We can't even assume that the general population knows what a healthy-weight person looks like anymore! I had family members concerned about one family member who lost weight down to a healthy weight. They thought that this person was starving or something like that, even though they were basically the only person in the family who had reached ideal weight.
I know I ranted for a long bit here but I think the USA in particular is long-overdue for some really major reforms surrounding the food industry. There need to be serious restrictions on advertising, portions, and macronutrient transparency. Bad food should be treated a lot more like cigarettes. For example, if I could make all the decisions I'd make it illegal to mass-advertise any food item that exceeded certain daily value macro-nutrients like saturated fat, sugar, calories, etc. Sure, you can sell them, but you can't put them on TV.