On the subject of ground transportation

One thing I wonder about though, is the wheel. Why are there no animals with wheels? It may seem like a stupid question to the layman, but to an engineer, it's a most puzzling matter.
Why do creatures (actually, creature may not be the most appropriate question here, because a creature has to be created... never my mind) have legs? A wheel is a much easier means of transportation, no?

My argument is based upon the fact that a wheel is so much simpler then a limb with a multitude of pieces. Oh's well, similar to the ornithopter, only few men bother putting the effort into making a machine that walks instead of rolls. For the rest of us laymen, we shall stick to wheels.