For the same reason they left out a lot of other minor characters and scenes. They seem to focus almost entirely on the plot-necessary things, and leave out everything else. Also, I'm imagining that her being a metamorphmagus could be a pain to do - though obviously they could avoid that just by making her keep a favourite appearance.
^^Which one, the one in Chamber of Secrets? I thought that'd been included. But it's been years since I watched the films so maybe I've gotten mixed up.
