I travel all around the US a lot for work (or did, pre-covid) and all the way down to your fast food restaurants all the way up to fancy $500/plate places, I've never once found it difficult to eat a "standard meat-with-vegetable-without-sugar-added". I would say that 80% of my meals are exactly that. I've never once been to a single non-fast-food restaurant where they didn't have multiple different kinds of vegetables on the menu.
There's certainly an issue with portion size at most US restaurants, but I've never found it difficult to get a plate of plain roasted vegetables with lightly seasoned fish, for example.
>It looks like this is hard to do in the USA: you don't easily find, for example, yogurt without sugar added.
Yogurt in particular is not really popular in the US, so you will not find it in many restaurants. You can, however, find packaged sugarfree yogurt in most convenience stores and certainly every grocery store if that's your thing. Other than yogurt, there are plenty of sugarfree snacks widely available. Starbucks is on every corner and sells plain vegetables, plain fruit, plain eggs, etc, for example.
I suspect your complaints are mostly rooted in simply not being aware of certain brand names or which stores are known for what type of food, because all of the things you mentioned definitely are available as long as you aren't going to McDonald's or Five Guys for every meal.