When you think about the main purpose of mitochondira (to produce energy; an easy idea how to remember the function of mitochondria in our bodies is to imagine them as energy factories or power plants) it becomes easy to understand why.
Animals (including us) move a lot more compared to plants. For movement you need energy and energy needs to be produced in our bodies in order for us to be able to move. Therefore, we have mitochondria that are bigger. Not only that, animal cells usually also have more mitochondria. All of this is probably done to keep up with the increased energy demands that moving organisms have.
