Have you ever wondered why horses have skinny legs?
What's the use of a skinny leg?
The answer is Running. The skinniest lower legs belong to the fastest runners. Think of how a deer can move, as opposed to a tortoise. Why is that?
It's because when you run, or indeed walk, you are constantly swinging the bottom halves of your legs. For humans, one foot is stopped and essentially resting on the ground while the other is moving twice as fast as your overall speed.
Then that foot stops and the other one accelerates. And so on. Thinner, lighter things are easier to accelerate than heavy, massive ones. So if you want speed, put the muscles up high and the skinny down low.
If you really want speed, you could go even further and pare down the foot, to decrease lower leg mass even more. A few million years of evolution and you'll wind up with...you guessed it. A hoof.