Because, pamann, what was a seat fight has now become a cabin baggage fight. Anything to make passengers move fast and keep the turnrounds short.
Plus herd instinct, plus habit.
Plus often you just cannot avoid it, because you are forced to join the back of a queue by a closing gate. Try and stay seated upstairs in the corridor at AMS and you will be barred as the gate closes, so at STD-30 you have no option but to move on and join the standing throng.