This question is rather silly. You can ask such a question about everything in the Torah. That fact that a blemished Kohen cannot duchen is problematic to you but the fact that a Levi or a woman cannot duchen is not problematic? The entire concept of kehuna is not a Meritocracy to begin with, it is passed down from father to son. So why does the superficial suddenly bother you?
(btw, today our kohanim are in nearly all cases not true kohanim but that is a topic for a different thread)
Daniela: How does what I say further your point? It is completely contrary to what you said.