Because you need to have the right air flow.
The duct work will depend on the square footage of the house plus it needs to match the size of the system that is pre-existing or being installed.
You need to have a licensed contractor that knows what he is doing
When the HVAC system is sized correctly, you must have a good delivery system for the conditioned air and return air. These systems work off of two types of pressure in the system (velocity and static). You should have approx. one cfm of air per square foot of livable space. The best duct work for delivery is rigid sheet metal, but flex duct is an option. You must have a contractor that understands heat loads and air delivery systems. In short, a perfectly sized HVAC system with a poorly setup delivery system will not properly do its job. And please change your filters because that also creates airflow issues.
Air flow is really important. If the air flow isn’t right nothing is right. Under sizing duct work either the return, supply or both, crushed ducts, hard turns, excess sagging duct work is very common. Also sizing a system based on square footage is not ideal. A heat load calculation should be done to determine the correct size system.