Yeah, unless a lot of those people are completely and totally tone deaf, and everyone that has ever listened to them are as well, they HAVE to know that singing just isn't the reason they were put onto this earth.
Some were close enough to possibly think that they had a shot, but many were so far off key that they might have just as well be singing in random keys with every other note.
But yeah, you would think that there would be one or more filters for those people to pass through before ever getting in front of the judges. Otherwise the judges would be wasting their time with not only completely talentless people, but also those just out for cheap thrills by making complete fools of themselves on purpose.
But I guess some people want their 15 minutes of fame, no matter how they go about getting it.