There is a large thread about this subject in the disABILITIES Forum. It breaks down to several reasons.
Unlike many chain restaurants most Disney meals are actually locally prepared and sources of food change often for a particular location.
For an example, a slice of whole wheat bread can have major differences depending on the bakery used, even if the slices weight the same and are the same size.
For a buffet style meal there is no portion control, so they cannot give counts for a "portion".
The main reason a chef will be available to explain ingredients is to assist a person in determining what foods are safe if they have particular allergies. Similarly, the quick service locations managers are similarly trained, although there is a book available to Guests at each location that gives ingredients.
But again the actual counts of calories and/or carbs in a meal depends on a lot of factors, all of which can change; only the list of ingredients tends to remain stable.