One of the missions of those helicopters was the immediate evacuation of personnel in the event of attack against Washington D.C. That requires the speed and ra

One of the missions of those helicopters was the immediate evacuation of personnel in the event of attack against Washington D.C. That requires the speed and range that a helicopter can provide but cars can't. To do that mission requires training and practice flying through the area, because as everyone in the military knows, you fight like you train, and so they need to be flying through DC regularly, both day and night. As for DCA itself, yeah, there have been people trying to close it for decades- that was originally the plan once they opened IAD in 1962. But DCA was so close and so convenient to the city and IAD (and later BWI) are so far away that no one has really wanted to pull the trigger on closing it, even as the airspace got more and more complex after the September 11th terrorist attacks.