Specifically, its effects are permanent. The only reason the ghosts can come back so quickly is because Prime Evil has built a high-speed tunnel (the Scareway) between the dimensions. When the Ghostbusters' fathers did their thing, this tunnel did not exist, and ghosts had to wait for a hole to open up, resulting in a bottleneck effect. This tunnel can also open anywhere and anywhen, which is why the heroes need to time-travel.
Prime Evil's trying to damage it, and the Ghostbusters don't really think about altering the timeline themselves: they show up, bust the ghost, and get out of there. Their costumes are equipped with perception filters and Translator Microbes, which accounts for just about every historical figure treating them without the least suspicion.
Tracy seems to be "modified" in intelligence, so it's not implausible that Zero, the unseen boss in the 1970s show, did this and sent Tracy back into time. Why modify a gorilla, though? Why not just enlist a human being? There's an explanation in the '70s show: Tracy's animal side is just intact enough to render him (her?) immune to specific forms of magic—a witch's "glamour" leaves Spenser fooled but not Tracy. Zero must have figured that this would be useful in dealing with ghosts.
Assuming Voldemort isn't already dead, his dark magic will consume him one body part at a time. He will thus be forced to replace each part with a bit of machinery. If Voldy is already dead, then his spirit is inhabiting a skull and other assorted machinery. Also, having to lead a horde of nincompoop ghosts in a succession of take-over-the-human-world schemes and failing each and every time to heroes who are barely more intelligent than his henchmen is the perfect Ironic Hell for him.
- First, and most simply, there are so many people buried in the onsite graveyard that it's a hotbed of ghostly energy (or PKE if you prefer). Alternatively, something REALLY bad happened there—bad enough to cause this effect.
- Second, the mansion is built over a crack between our world and the Spirit World. (This makes the most sense: the ghosts use it as a temporary oasis.)
- Third (and this is stretching), Ivo Shandor had a hand in its construction, and it's been drawing in spiritual turbulence ever since.
- A live action episode showed Eddie Sr. mooning over a photo of a lady truck driver named Mable. Since both guys seem closer to their dads than their moms, maybe Jake's mom also had a job that also took her away from home a lot?
- The old haunted castle was one of the last remaining doorways to the Spirit World, and, with every successful bust, the doorway closed a little more. Zero simply had the Ghost Busters mop up the last few ghosts. With the last ghost gone, all would be well until 1986.
- How else do you get a blue eyed blond with the name "Kong"? It's not exactly a Swedish name. Noah emigrated from Sweden to the US when he was a very young boy, but his parents didn't survive the trip. A Chinese couple had been hired to work on the ship as cooks and felt sorry for the orphan. The Kongs took him in as their son. Noah Kong remembers them fondly and occasionally burns Hell money for them. Jake Sr. has some fond memories of Nai-Nai and Ye-Ye but they had passed on from old age when Jake Jr. was very young.
