Why do we have kneecaps on our legs but no “armcaps” on our arms?
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Knees go back and forth. Elbows go back and forth, side to side, and twist.
The kneecap provides stability in the back and forth motion, but you need more versatility in your elbows.
Less ELI5: It’s the same principle as your hips and shoulders. Unlike the knees and elbows, hips and shoulders are the same type of joint (ball and socket) yet you have far greater range of motion in the shoulders over the hips. This is governed by structures outside the actual joint itself (huge muscles around the hips, more tendons puppeteering the shoulder). The downside is that the more versatile joint is also less stable. This is why you hear about rotator cuff injuries more often than you do hip sprains.
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