Heathrow (Third Runway)

I have not made the case that stopping the third runway would end up with fewer flights. I have made the case that the Government have accepted for roads. The Government have accepted that, if more road capacity is built, it becomes filled with vehicles, making journeys that would not have been made. However, the Government do not want to accept the case on aviation that, if more airports and runways are supplied, there are more flights. Why do they not accept that argument? I fail to understand the logic that the Government adopt.