Ghosts Confuse Me

So, somebody please explain this to me. Why don’t ghosts just fall through the center of the earth and lose themselves in the abyss of raw space out on the other side? I mean think about. Ghosts can walk through walls (supposedly) and they can’t pick up physical objects, so why would their ghostly feet keep them on the surface of the earth? Or how could they climb stairs? Wouldn’t they just fall straight through? Or is it because they float, and never actually make contact with the ground?