The reason you only encounter ghosts who are malevolent and attempt to kill you/your party isn't just Rule of Scary (as Phasmophobia would both become boring and lose its scary factor otherwise). The harmless ghosts do not attempt to harm anyone, so they may go completely unnoticed or are considered a non-priority by the company the players work for. Furthermore, non-hostile ghosts also probably wouldn't require such a careful investigation before the exorcism; the exorcists can just take their time, narrowing down the possibilities at a more leisurely pace or even through trial and error, since the ghost isn't actively trying to kill them. An actively hostile ghost would put them on a much tighter schedule, since they've got to figure out what they're dealing with and perform the exorcism before the ghost kills them all. In the case of a hostile ghost, separating the investigation and exorcism into two separate phases helps minimize the risks associated with the latter, since they can get in, do the exorcism, and get out as quickly as possible.
The Asylum and Brownstone High School maps are both derelict, abandoned structures that are not used anymore. So why is the company conducting paranormal investigations there? Because abandoned buildings do sometimes end up being "used", by all sorts of folks from homeless people to brave urban explorers, and as a result someone has paid for them to exorcise the ghost so they are able to "use" the building without the risk of a ghost being angered enough to attempt killing them. Even if the buildings are slated to be demolished, the ghosts are known to throw around objects — while not all ghosts can do this, the demolitions company won't take the risk that the ghost can, given that demolitions often involve explosives and/or big, heavy, and dangerous equipment that can easily be lethal if the ghost messes around with them. There's also the possibility that the ghost is bound to the land instead of the building, and that demolishing it without an exorcism would just anger the ghost.
Fridge Horror
The manuals that the ghost hunters use seems suspiciously well-documented and detailed. Given just how lethal the spirits in this setting can be, one wonders what kind of "research" had to be done to compile it.
Likewise, the fact that some of the tips for the ghosts' strengths and weaknesses are vague, and in some cases incorrect, makes sense from a story aspect. How few ghost hunters would, for example, be able to figure out that an Obake can shapeshift and leave behind unique evidencesnote A six-fingered handprint? Furthermore, how fewer would be able to live to explain what they wrote down?
The seeming saturation of ghosts even in environments such as suburbs where one typically would not presume them to be present with regularity, and the fact that the safehouse itself seems to be infested, paints a surprisingly grim picture, especially the later. For what is apparently a component of a larger ghost hunting organization with access to exorcists, the fact that the players seemingly live in a run down garage with at least two ghosts skulking around it and the fact that exceptionally basic items cost an absurd degree along with the low value of an investigator's life insurance would suggest that the situation is less isolated incidents being investigated and more some kind of ghost apocalypse where society, or at the very least your organization, is struggling under the strain.
