Because your seat reclines into the space of the seat behind you. Airline seats are very, very close together, often only 14 inches of space. If you take up 3 of those inches by reclining your seat it makes evacuation for the people in the row behind you slower in the unlikely event of an emergency. But, since those emergencies can be super dangerous and slowing the evacuation could kill people, you’re not allowed to recline your seat when the plane is close to the ground and the warning time of an emergency might be very short.
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