The main strength of the leg is a lengthening push (quad). The main strength of the arm is clenching pull (bicep). An elbow cap wouldn't help with a pull.
Presumably, that pull strength (lifting things, climbing trees, gripping parents/babies) is evolutionarily more important than the push strength (punching).
Or rather, it is more important that the legs have a good push. Even in other mammals (most which have knee patellas), the leap is a push initiated from the hind legs.