It's just how they stand. Like us, for example. We stand on two legs, and they are probably saying "Why do humans stand on two legs?" It's just like asking "Why do dogs stand on four legs?" Please recommend me!
If you are finding that your legs get so numb that you are unable to stand, you should see a doctor. It is normal to get numb legs from sitting for long periods of time but you should be able to stand up.