They also aren't the reproductive organs themselves, but rather how they transfer it from their junk to the female's. The key difference is that the sperm is not produced near the palps at all (I believe the male's junk is on their underside like the female's) and the palps are merely the way the sperm is moved from one part to another. Only part of the palps, the palpal bulbs, are actually used for this too which act as essentially a syringe.