The apparent scarcity of ghosts is inconsistent with the abundance of dead people. There are two theories that dominate scientific discussions about why this may be. Both are horrible prospects.
The first suggests that ghosts are massless. If this is the case, then gravity cannot aid them in staying grounded. Thus it takes great effort to stay on the surface, and this effort requires skills that newly dead ghosts are unlikely to yet be familiar with. The outcome therefore is that they will drift away from earth and find themselves floating through empty space in eternal solitude.
The second theory suggests that ghosts do in fact have mass. Ghosts are known to be able to pass through solid matter. It follows that if gravity is acting upon them, they would have to resist that force in order to stay above the ground. Again, resisting it may not be easy for new ghosts, and they would find themselves phase into the earth. Slowly they would sink, until they reached the Earth’s core. Here they would join countless other ghosts, trapped in a screaming ball of horror, forever unable to return to the surface.