In this video I talk about three main and obvious reasons.
There is also one more quite important reason concerning such high-developed, clever and difficult to keep birds, such as Corvidae (latin). Especially, when it is large Corvidae, like black raven.
Corvids group always have certain hierarchy to which all birds in group follow. And one of signs of the steepness and greatness of the leader is the highest place on the perch. Other birds take seats under the "boss", in decreasing order of the hierarchical importance . When you allow "crow" sits down on your shoulder or (even worse) head, you thereby officially put it on the same level of importance. At least, you do her authority equal to yours, and at most, you give it a signal that you are no longer a boss. And try to prove otherwise. Eventually you'll succeed, but if you will do it often, without understanding what is happening from a bird perspective, you will just drive it crazy.
It doesn't concern owls, they don't care where to sit, the main issue that paws didn't slide off. Don't ask about parrots, I am afraid of them, avoiding them and not interested in them.