Ahhh, yes.
The age-old question. (Aviation Age, at least.)
One might similarly ask:
Why do so many people stand crowding the gate when they aren't due to board for another 20 minutes?
Why do so many people race eagerly just to stand in the aisle for 20 minutes until it's their turn to de-board?
Why do so many people ignore the instructions to put their smaller item under the seat?
Why do so many people place their bags sideways right in the middle of the overhead?