They don't, there is an audition process and they let some people through that suck or that would make decent tv. They narrow down the people who go through the tv judges a good good bit before they are allowed to be on camera and etc.
My brother had a classmate give it a try. She waited around for a few days to get a "sorry". Who knows how many the main judges even get to hear. I guess I would never waste my time embarassing myself...lol