You would be stunned at the absolute carnage a belt outside causes to the airframe in one circuit.
Just as a side note. I fly around with the window open and sometimes without a door............... sort of negates the hatches and harnesses check. and besides, If it had to integrated into a check list item, Harnesses should be at TOD as it is a legal requirement for the PIC to be wearing a harness/seat belt at or below 1000 ft AGL.
