I'm not positive, but pretty sure it is the same reason carryon has to be placed under the seats. In case of an evacuation (which is most likely at takeoff or landing) you need as little obstruction as possible to get out of your row and to the exits. Same reason the lights are dimmed, so we are accustomed to natural light and can escape if there is a power loss.

Maybe some FAs can confirm this, but i'm pretty sure this is why