why doesn't MFP deduct calories when you do weights?

Add "Strength training (weight lifting, weight training)" to Cardiovascular to get estimated Calories Burned added to your Diary. Please note that the Calories burned for Cardiovascular exercises provided by MFP are based on published metabolic equivalent of tasks (METs), which are estimates for a general population and may differ for you as an individual. If desired, add individual strength training exercises, such as "Biceps Curl," to Strength Training to have a log of sets, reps, and weights as individual exercises.

Please see these articles in the list of articles on this topic in the MFP Help pages...
myfitnesspal.desk.com/customer/portal/articles/11170-why-don-t-you-calculate-calories-burned-for-strength-training-
myfitnesspal.desk.com/customer/portal/topics/455842-exercise-diary-and-exercise-database/articles