what's to stop it from appearing to each and every person when they have reached a relatively mature age, and explaining which religion is the right one to follow?
I think it has something to do with free will and forcing people to follow God. If God just spoke to people like that, they would, presumabably, know that Christianity is the right religion, but that would not always make them want to follow it. They might feel forced into it and not truly repent of their sins.
If you say there no absolutes, I ask you, are you absolutely sure?