The fetal posture will save your life when it is in danger and it is easier to do if the chair is erect.
Third, in case of emergency, the aircraft has trouble, the back of the chair upright will help a lot . If the seat is straightened, the rear seater will curl up in the fetal position faster, easier, without collisions, injury with the front seat back.
In addition, the upright chair helps people move more quickly and easily outside.