tdracer, as I recall - and its been many years since I studied this - the Germans knew this. The airfields were rather well defended and any attack on them would come at a high price. Also, bearing in mind towns were hard to find, landing bombs on an airfield and putting it out of commission would be a tall order to say the least. Finally, the Luftwaffe had a small number of key locations in the South East of England to target. Shutting, and keeping shut airfields across Germany, France and the low countries would be a gigantic task - especially as the aircraft could always just operate from grass fields anyway!