Why do you think that is?
My theory is because most officials (all sports) think they are better than they really are. It takes a decent amount of ego to do what we do and if we didn't believe in ourselves, we wouldn't be very good at all. So as I've gotten older, I have found that there is a very thin line between having the confidence that it takes to handle yourself in the heat of the battle, paired with the humility needed to know when to admit that you might not be ready for another level. If we are honest with ourselves, we have all felt that we were better suited for a big game that went to someone else (via the assignor)...and felt some level of frustration with not being given that opportunity. On the other hand, I don't know very many guys who tell war stories about screwing up a game because they were in over their head.
It is the assignors job to make everyone happy...a position that is nearly impossible. If he doesn't send guys that are good enough, the coaches are not happy. If he sends officials to games that aren't challenging and competitive, they aren't happy. And if he is an official himself, he is further scrutinized for any game he takes for himself regardless of his ability. The worst assignor is one who is on a power trip and is not approachable. Everyone deserves to know where they stand.
If you take this job OP, always be approachable and honest with people. It will help you in the long run if people know that they can trust you to