Because the (human) parents of humans don't have four legs. The basic four-limbed design has been around for ever. Humans co-opt the anterior pair for hands, so that only leaves two for available for legs.
Of course koalas have legs. Koalas have four legs, all equipped with very sharp claws. Without legs, they would be unable to walk, climb trees or feed, since they use their front paws to grasp hold of eucalyptus leaves.