Quick remark: Sugar, as other carbohydrates, is cheap. It also tastes well, objectively (as much as taste can be objectively measured).

So if you go for industrial-level food production, you end up with a higher proportion of sugar than what would be considered healthy. Sugar per se ist not bad at all (try to have an apple without it ;)). But our economy demands cheap, somewhat tasty, calories. Hence we end up with a lot of sugar or carbohydrates in general.

Any diet that rules out carbs works well because of that, not because of some intrinsic property of carbs.

As a French person you would probably never consider giving up on a good croissant or baguette, but in France these things are expensive. I doubt anyone gives their kids fresh croissant before school every day.