In the recent article interview with Marmel, he talks about how the Ghost Zone can be seen as ¨a dimension of unfinished business¨.
This would mean that some of the characters we see in the series exist as ghosts because they have some unfinished business they left in the Human Realm and they will probably going to keep existing until they find a way to fulfill it.
I think this would mainly apply to the characters ghosts who look the most human like or have a confirmed backstory- such as it is the case of Ember and Sidney- are the ones that have unfinished business
There are other ghosts that likely were just born in the Ghost Zone and part of the ecosystem, maybe a mix of certain human emotions and memories that were there for centuries and mixed with ectoplasm.
This aspect also can be easily tied to A Glitch in Time graphic novel, where it was revealed that a lot of the ghost power comes from emotion. A lot of ghosts have some form of ¨emotional drive¨ that is connected to their powers.
So this would turn the Ghost Zone into a place where souls with unfinished business end up going when they die in the human realm and can stay that way for who knows how many years.
