I don't think it makes sense for people to believe in ghosts, but not believe in God, for one major reason: How can people have souls without God? According to science, it doesn't make sense for one to continue existing spiritually after physical death.

A friend of mine once used the argument that, since physical energy cannot be created or destroyed, the evidence of ghostly beings is just the leftover energy of the deceased, but that's not how this energy works. How can leftover energy make conscious decisions and deliberately "haunt" the living? It can't. What happens to the energy of dead people, is simply that it gets consumed by whatever organisms decide to eat the flesh, thus, transferring that energy. It doesn't just become a free-floating blob of energy.

Alternatively, spiritual energy is different. It does exist with a conscience and can haunt the living. The idea that spiritual energy can exist without God doesn't add up because, when strictly evaluating the circumstance from a scientific point of view, spiritual energy isn't a real thing. Yet, we claim to experience hauntings and spiritual scenarios on a purely realistic level.

If there is such thing as spiritual energy, that means people, in fact, do have souls and God is real. Here's a kicker to the idea that ghosts are actual souls of the deceased. "Just as man is destined to die once and after that to face judgment." (Hebrews 9:27)

This does not mean that the spiritual encounters we experience are false, however, they're not what a lot of people think they are. Since man dies once and then is judged, the souls of humans don't walk the earth. So, who is the one in charge of hauntings?

I typed into Google, "What does the Bible say about ghosts?" I got this result.

Demons. The answer is demons. Satan is the father of lies. So when the ghosts of proclaimed "loved ones" appear, it's not actually the souls of our loved ones. It's actually imposters, AKA, demons, under the impression that, if they can fool us into believing they really are our deceased grandparents, they can string us along and tear us away from God.

This is actually why some people believe that there are spirits who don't seek to harm. The thing is, they all seek to harm. I like to think of them as being on different levels. For example, a demon may not try to physically attack us or scare us right off the bat. Level one is to gain trust. Therefore, they don't aim to scare their victims. This opens doors so that, further down the line, their victims will be easier to manipulate. (Predators offer candy to children before they attack.)

"Be alert and of sober mind. Your enemy the devil prowls around like a roaring lion looking for someone to devour." 1 Peter 5:8

If a demon can gain the trust of a medium and make them believe they can actually contact the deceased, it really opens a whole can of worms, which may not seem harmful at first. But, in reality, it causes vulnerability for the medium and anyone who seeks out the medium. This could very well be why God has warned us to not turn to psychics for help.

“When you come into the land that the Lord your God is giving you, you shall not learn to follow the abominable practices of those nations. There shall not be found among you anyone who burns his son or his daughter as an offering, anyone who practices divination or tells fortunes or interprets omens, or a sorcerer or a charmer or a medium or a necromancer or one who inquires of the dead, for whoever does these things is an abomination to the Lord. And because of these abominations the Lord your God is driving them out before you." (Deuteronomy 18:9-12)

So, if ghosts exist, souls exist. If souls exist, God exists. If God exists, the souls of man do not remain on earth. If the souls of man do not remain on earth, the ghosts we experience are demons.