When a car moves along the highway at constant velocity the net force is zero why then do you continue to run your engine?

When a car moves along the highway at constant velocity the net force is zero why then do you continue to run your engine?
The key point is that the net force is zero. This is not the same as having zero force acting on it. The forces of drag and friction are always acting on moving objects, and the engine generates a force through the wheels that balances out this force, keeping you at a constant velocity. If the engine was stopped, this counterforce would no longer be in effect, and the unbalanced force of the drag would eventually slow the car to a stop.