Because any food and its Points value is based on how many calories it has per serving...and then can be lowered or upped based on the fat or fiber content of that particular food.
A cup of soup is not just cooked carrots. It contains broth, other veggies, and possibly rice, beans, meat, or noodles, depending on the variety. The total calories/fat/fiber per serving are what make up the amount of Points...NOT what the food is, and whether or not it is an ingredient.
A measured cup of cooked carrots is going to have a different calorie/fat/fiber level than a cup of soup with 5 peces of carrot IN it.