Why are airplane doors and windows completely sealed?

Modern jet airplanes fly at high altitudes, where

there is less air and therefore less oxygen. Aircraft cabins are

pressurized so that pilots and passengers can have enough oxygen to

breathe. There is a high air pressure inside the cabin because

outside air (and oxygen) is forced into the cabin by the engines in

order to pressurize the inside of the airplane, and this gives

people enough oxygen to breathe. This is only possible because the

airplane is sealed; if the Windows and doors were open, the engines

wouldn't be able to pressurize the cabin.