Funny how you completely left out (sweet)potatoes, paprika, carrots and other vegetables with as low as 2g of protein per 100kcal

Other than potatoes, those are all low calorie foods that are easy to just ignore when meal planning. You can eat a 2 kg (4.4 lb) bag of bell peppers/paprika, roughly 15-20 peppers, and it's only 600 calories.

Wheat products being pretty close to begin with sounds nice but it still means they are too low on protein and those definitely make up a large portion of most peoples calorie intake.

They are definitely more than enough for most people. The example I used, of 5g protein per 100 calories, is a fairly tight example of a bodybuilder on a calorie restricted cut, and probably twice that of a person with more typical needs.

For someone who is physically active and has a higher caloric requirement, like a typical 180 lb (82 kg) athletic man, their maintenance calories are 2800 and their protein needs are around 100g (with health issues possibly showing up below 70g). If they're actively bodybuilding they might need 150g of protein, but they're also going to be physically active enough to have a higher calorie budget. And for most people, there's no muscle building/retention benefit of getting more than 0.8g per pound (or 1.8g per kg) of body weight. Basically, the 5g per 100 calorie budget applies to a pretty narrow category of people who are trying to optimize for something specific rather than just people who want to live their lives.

And even for those vegan bodybuilders, it's not that hard to just supplement with pea protein, eat some processed proteins (like tofu), not that different from omnivore bodybuilders who use lots of whey/casein protein isolated from dairy products. Or they can go get those protein supplemented pastas and breads and just eat normally.

For a normal 200 lb (91 kg) man who isn't trying to be a competitive athlete, 80g of protein on 3000 calories is enough to stay healthy. So when budgeting 2.6g of protein per 100 calories, at that point things like potatoes and carrots and bell peppers are already staying ahead of the curve.

Obviously less choice makes things harder. But for people who are actively trying to optimize their diets for body composition, the supplements are useful regardless of whether you eat meat or not.