The chance of an airplane hitting major turbulence unexpectedly, while pretty small, are much higher than the chance of some random debris falling from a buildi

The chance of an airplane hitting major turbulence unexpectedly, while pretty small, are much higher than the chance of some random debris falling from a building while you are passing by. Ultimately it's all weighing the inconvenience against the possible safety improvement: when you are sitting in your seat in a plane, the seatbelt is a very minor inconvenience. Wearing a helmet all the time is a much bigger inconvenience, with a much smaller benefit.