Most companies don't want to let you off the hook until someone has accepted the position. In a well organized company, once someone has accepted the position, they will let the other finalists know. This can also happen during each round of interviews.

However, if you are a hiring manager and you are short-handed and overwhelmed, letting people know they didn't make it to the next round is likely the last thing on your mind. That is where the follow-up helps, reminds you to let them know if they didn't make it so they can move on.