It’s a rule, and you obey the rules or the flight attendant tells you off and you wouldn’t want that.
Why does the rule exist?
Newtons first law of motion states: ‘An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.’
In this case, the armrest is that object. It is attached to the aircraft which is typically travelling at 135-155 knots for take off and landing. If the aircraft was to come to a sudden halt, the armrest would swivel forward at the same speed as the aircraft was at prior to halting. I.e, the armrest could slam into your side with a force as if it was travelling at more than 135 knots.
So it’s very simple really. An upright and free armrest could knock into you and do some very serious damage, given the speed of flight.
He also explained that safety is the top priority of the crew. That may be why they’ll be terse with you:
Great care is taken to even the smallest of details as far as aviation safety is concerned.
By all means, raise the armrests to get in and out of your seat, but make sure it is down for taxi, take off and landing.
The same goes for your window – if the blinds are down the crew cannot see the outside of the craft.
In an emergency, the crew may need to report the condition of the plane to the pilot. In the case of this happening, they don’t want to be waiting for you to pull up your window blind.
Plus, if you have to make an emergency landing at night, your eyes will already be accustomed to the low light, meaning your evacuation from the plane is safer.