Exactly. In my experience, it's far more common to not receive a letter at all. As disappointing as it is to receive a form rejection letter, not hearing anything is worse.
I'm guessing in some cases they've made a hiring list and you're on it. Just not at #1. They don't want to reject you and then find out everyone above you on their list has taken another position.
On the other hand, most of them are probably just jerks.