Diabetics have to manage insulin levels. They manage their carb intake because of this. It has nothing to do with managing their weight. With respect to obesit

Diabetics have to manage insulin levels. They manage their carb intake because of this. It has nothing to do with managing their weight. With respect to obesity you might consider what you just said: carbs are abundant and cheap. Therefore, people tend to eat a lot of them. Significantly more than they should. Carbs also do not tend to induce a level of lasting satiety that protein or fat does (note, 1g carb = 4kcal, 1g protein = 4kcal, 1g fat = 9kcal). So not only do people tend to eat a lot of carbs but they also tend to eat them more often because they don't feel full for very long.