Let me reassure hiring managers everywhere: If I come in for a job interview, I want the job. Maybe someone, somewhere, doesn't really want to work and goes job hunting only because their dad makes them, but in general, assume that if I've chased the position this far, it's because I want it. Sending a follow-up email isn't some significant extra effort from the best among us. It's extremely easy to do (once I know I should). Dragging myself all the way to your office for the interview was the hard part.
A thank you note doesn't show someone's well mannered; it shows they know your personal view on what good manners are. And to be fair, that's true of most "manners," but the writer isn't emailing you because they're courteous. They're emailing you because they want news on how they did and want to plug themselves again. I want that too, only I thought pestering you so soon was bad manners. If I could, I would definitely end every interview by saying, "Oh yeah, and I really need this job. Let me stop in the elevator for a couple minutes to reiterate how amazing I am, and also, did I ace the interview? Can you tell me?" I just assumed I wasn't supposed to.