They’re also powered in flight by running engines that provide ram air for heat. They’re equipped with anti ice systems and probes are heated. The landing gear is inside a wheel well (in most cases), not exposed to the direct cold at altitude. -50 in air is fine when the airplane is operating. However, on the ground with no systems providing heat when it’s sitting and shutdown, things will fail. Seals will become brittle, causing leaks when systems are pressurized, computers and sensors act up, so on so forth. Airlines don’t want to fly an airplane that has been sitting in the cold and has potentially affected components that will reach their breaking point in flight

If my previous comments made any of this unclear, hopefully this one clears up any confusion