I was surfing on the internet for a long time, but I couldn't find the answer to my question: Why do we have only one elbow per arm and one knee per leg, not multiple elbows or multiple knees per limb?
The simple answer is: our ancestors did...as did our ancestors ancestors, and so on. We are not optimally designed, but rather, we take what our ancestors give us, and work with that. The horses for example. They have added another joint by essentially extending their toes, and walking on them, which gives them an advantage in running. It is possible to modify the skeleton, but even in this case, the original is modified, not fundamentally changed.
We are the way were are solely because of evolution. The only reason we do have a single elbow per arm or a single knee per leg is only because it was advantages to have these bendable joints. Long before humans existed little organisms with small sticks as legs and arms probably roamed the Earth, but there must have been a single time where a mutation came and gave and organism these joints we know have. These elbows and knees are obviously very helpful and preferred. Also the main reason we don't have more than one elbow per arm would probably because no random mutation has come about and on top of that there is no reason we would need it.