Because it’s not carbs that make you fat, it’s overeating, and it’s the quality of your food.
Italian and French breads are very simple, they are usually made of four ingredients only (flour, water, a pinch of salt, and yeast or sourdough). If you open your cupboard and check the ingredients in your bread sliced bread you will probably see that it contains lots of sugar and fats and other ingredients. Italian and French breads are nutritionally better than that loaf of whole wheat sliced bread you keep in your cupboard.
Secondly, it is true that we eat lots of bread, but our meals are not carbs on carbs with a side of fats as I see often the Americans do*. An Italian or French meal contains lots of carbs, but very little sugar, loads of vegetables, some fruit, and some proteins. Besides, both us and our transalpine cousins (Salut, ehn!) have portion control and tend to eat at meals only, with very little snacking. We have an eating culture, in other words. Eating is when the whole famlies meet, it is when you send quality time with your loved ones and friends. Finally, both Italy and France have a walking culture. We walk or bike to places, we use cars only if strictly necessary.
So, sure, we eat lots of bread. We still eat less carbs than the Americans and tend to eat better quality complex carbs, lots of vegetables, stick to our meals, tend to prefer quality over quantity, and are more active.