system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have kneecaps on our legs but no "armcaps" on our arms?
gpt
Kneecaps allow to have a lot more torque when the leg is fully folded.

This is great because when you are squatting at the point of sitting on the floor, you need a lot of strength in your knee to be able to stand up with just your legs. You may notice it's pretty hard with just one leg, this requires a lot of torque. The kneecaps are crucial in allowing for this.

So why not having this on the elbow? Well, those have a cost: they make the articulation less precise as there are more moving components. So elbows have fused the cap with the forearm, that's why you forearm is pointy. It's not as effective, and you will have really little force when your elbow is fully bent, but in exchange you get higher rigidity and precision on your arms. It's likely that without this rigidity we would be much worse in term of accuracy.

But in general it's hard to find a definite reason for not having something. Sometimes it's just nature haven't yet stumbled on the solution and the current mechanism is good enough.