Answer:
Plants make their own food from sunlight, water, and carbon dioxide (part of the air). Making food like that is slow, so if they moved around too quickly, they'd starve to death. So, though they do move, it's usually very slowly.
Animals get their food from plants or from other animals. Moving around is a way for them to get more food, not just to use it up. It's also a way to avoid getting eaten.
(Even plants which appear to "eat" food, such as a venus flytrap, are actually only getting vitamins and minerals that way, not extra energy.)
