I usually hear the prerecorded sentence "Doors are closing" inside elevators. Now I am curious to know why can't this sentence be "Doors are getting closed" instead of the one which is before-mentioned. Thanks in advance.
Doors are getting closed implies that somebody else is in the act of closing the doors. But that's not what the statement is trying to say. It's trying to say that the doors are performing the action of closing by themselves.
Also, "getting closed" sounds very awkward and ugly to a US English speaker. Usually if we want to say that somebody else is performing an action, we use being, like "The doors are being closed." We do use get to mean that something is changing state, or becoming something else, like "He is getting older" or "She is getting better at tennis", but "getting past-participle" implies that somebody else is doing it to you, like "The robbers got arrested by the police" or "The fish will get eaten by a bear if it is not careful."