Where do ghosts come from in the Sims?

Hmm... Interesting point...

One fun thing to remember is that when The Sims 4 was released, there were no ghosts. Sims that died were gone... Ghosts were added into game a month later.

Yet as the animation has been same for 10 years, there has to be a reason...

My guess? It's collecting the body which is turned into a spirit-like substance. But if we go through that theory that it's a soul, let's look into the Reaper career from Life & Death. When your sim reaps someone, they'll get their soul in their inventory which can be sent to the netherworld through the portal. Ghosts can freely come out from the portal to sabotage the workplace, so ghosts can always come out of that place.

FlamingoKickerThere are three planes of existence in the Sims 4: the physical world, the spirit world, and the netherworld. The Grim Reaper himself, as well as his workshop, exist at a sort of nexus between the three. It's like the "Wood between the Worlds," if you've ever read Chronicles of Narnia. 

When a sim has just died, their soul is temporarily trapped in their body. When Grim comes to reap them, he is separating their soul from their body and transferring them to the spirit world. A ghost in the spirit world is still connected to the physical world via their tombstone/urn, and can visit the living. The longer they remain in the spirit world without anyone strengthening their connection to the physical world, the more they want to go to the netherworld. Eventually they will go on their own. If someone in the Reaper career reaps a ghost who is already in the spirit world, they are sent to the netherworld. Once in the netherworld, they have no connection at all to the physical world, although they can still visit the Reaper's workshop.

Grim Reaper never sends these souls to Netherworld, so he may free them or they escape eventually and become ghosts, specters or skeletons.

It's not clear about skeletons but the souls may attach to their former body.

That is true.  The spirits do sneak back into the "physical" sims world from time to time and the Reapers have to go back and wrangle them back up again.  So it isn't like once they go thru the portal that's it for them.  It is a game after all so maybe I need to not think so hard about it?