Why is it necessary for airplanes to fly at such high altitudes?

Question

Commercial airplane Jets (i.e Boeings, Airbus') fly at altitudes of 30,000-40,000 feet during the course of their flight. I never have understood why it was necessary to do so. Why can't they fly at 4,000 feet or so? Wouldn't it be easier for a pilot to maneuver a more safe landing in the case of an emergency if the airplane was not so high in the air?

Answer 1

The atmosphere is thinner at higher altitudes, so there is less wind resistance, meaning the flight takes less time. Also, more importantly for the airline, it uses less fuel to fly at higher altitudes. That's the main reason anyway.

Answer 2

Mike is correct; at higher altitudes you also avoid mountains, small aircraft, and most severe weather. emergency landings are very infrequent now, but there would be many, many more if planes flew lower.