Some do, but relatively few want to. Many animal souls prefer not to get entangled in the complexities of human life and there is no need to, although the option is there. They can complete their karmic cycles just as well within the confinements of the animal kingdom. There is no value judgment in the real world of the higher dimensions which all living things are part of. So none is better or more evolved than another. Such discrimination is more a human thing. Animal souls, angelic souls, or whatever other kinds of souls are like the different races, not one race being better or more advanced or valuable than another race. I have heard horses talk more philosophy and wisdom than I have ever heard from a human being. So it is not important in what form you are, but who you are in what form.