JMO, but I believe they send out a lot of invites and spend a little more time personalizing them for the players they really want at the camps. Most of the players at a camp are there so that the players they really want to see have somebody to play against and so that they can make money off the camp.
Now if a player they didn't know anything about happens to stand out at a camp then they'll take notice. It is just a lot harder to stand out if they're not looking for you, especially if you're a position player.