Why do we have kneecaps on our legs but no “armcaps” on our arms?
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The forward extremities bear a far lower amount force and torque, even among quadrupedal species, than the hind limbs. The forelimbs are more for balance and agility than locomotive propulsion. The mammalian class of chordates are, generally speaking, like a rear wheel drive vehicle. All of the power is in the back tires, and the steering wheel is in the front. Birds of flight, conversely, rely on their fore limbs to propel them, while steering, in some part, with their tail feathers.
Now there are certain higher primates that exploit primarily arboreal habitats for whom brachiation is the preferred mode of locomotion. Even if this exerted a similar type of force on the limbs, which it doesn’t but let’s just say it does, it would take eons of mutation and natural selection to produce a skeletal analog to the patella.
Short answer, at no point along the evolutionary path, at not a single decisive node of branching on the tree of life, has there been sufficient environmental pressure of genetic survival to warrant or reward the growth of an elbow cap. It’s just not necessary. If there has ever been a mutation that presented this particular allele, it was of no significant biological advantage.
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