After Barbara says Beetlejuice 3 times, her and Adam are transported into Adam's model where Beetlejuice is residing. Why did they have to dig him out of his coffin? Why was he just not there already?

I had two thoughts on this. The first is that the digging was symbolic to their commitment to Beetlejuice and they had to prove that they were serious. But later on, Juno yelled at them for, "letting him out" which to me suggests that he was somehow imprisoned and their digging was necessary to break him free. But earlier in the movie she tells them, "he has been lurking around your graveyard" which doesn't sound as if he's been imprisoned at all.

So, why did Adam and Barbara have to dig Beetlejuice out?