My teeny tiny Beetlejuice Beetlejuice theory is this:
"Barbara and Adam no longer live here. We found a loophole and they moved on"
My theory/heardcanon is that it was raising Lydia that set them free. You know how it is with ghosts, they stay around because they have an unfinished business.
And Maitlands wanted to have a kid but couldn't have one of their own. But they were basically a second set of parents to Lydia and they watched her grow up and when she was finally an adult, they realised they have a kid, they raised a whole ass wonderful smart girl. And so they could move on <3 and I chose to believe this
(Also they're so t4t too me)