I have heard it said that ghosts, spirits, etc. are only here because they have unfinished business. I don't subscribe to this theory. A prime example would be repeaters.

What is everyone else's take on it?

I think there are countless theories on this including the one you have spoken of. I feel the majority are here because they are stuck in a routine that they are content with.We all know people who do the same thing over and over again. day after day and so on.Some are loners they are the people know one pays attention to they are there but at the same time they are not. a lot of spirits I feel still want to be close to you . Some still want to be physical. Some earth bound spirits don't realize they have passed on.Some found pleasure in being cruel . Some fear what is on the other side. some are waiting for judgement day and so on.

Thank you, Norman. Exactly.

There's also the theory that there are multiple worlds or dimensions that are parallel to ours, and sometimes, those dimensions cross planes (whether it's from magick or from another reason), and when they do, they can cause apparitions from the other world, which look and sound like ghosts.

There are also theories that combine space and time. If and when conditions are right, time folds back on itself occupying the same space that we (and they) are living in. For example, we may appear as ghosts or spirits to someone in 1948 while we are investigating a sight and communicating with them.

There are a variety of reasons to including waiting for judgment and/or being held here against their will.