The main reason is poor design and installation practices. All of the joints in our installations are sealed with mastic, making them leak free. Many duct systems are poorly designed due to lack of education and training. Many are the result of cost cutting measures. The duct system is like the circulatory system of our bodies. Instead of blood, we are moving air. If the air can not move properly, the system will be very inefficient, costly and will not deliver the level of year round comfort that you are seeking.