In Macbeth why are the scenes (and even Act 5 itself) so short?
You are correct in your observation, and this is particularly true when we think about other Shakespeare plays, which actually only contain one extended scene in Act Five in which all of the resolution occurs. However, let us consider the impact of these short, choppy scenes on us as an audience. There is a real sense in which the number and the length of these scenes help increase tension as we quickly switch from one scene to another, eagerly awaiting to see what will happen to Macbeth and how the prophecies will be fulfilled. Also, it is important for the plot of the story to give us the perspectives of a number of different groups of people: Macbeth himself and his confidence, his Lords and soldiers who desert him, and then the forces of Malcolm who are attacking. It was necessary to do this through chopping and changing to different locations. The nine scenes then in the final act serve to create tension and also chart the feelings and actions of different groups as the inevitable end closes in upon Macbeth.