system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do knees have kneecaps, but elbows dont have elbowcaps?
gpt
The kneecap enhances the leverage the quadriceps muscle has on the lower leg by acting as a simple lever on which the quadriceps tendon rides. This allows the quadriceps muscle to perform the powerful movements the leg requires. 

Simply put, the elbow doesn't have the same requirements for massive muscular power to be exerted across the joint. The triceps has enough leverage the way it's configured to meet the needs of our upper limbs without another structure to enhance it.