Why don't ghosts sink through floors?

So perhaps I watch far too much science fiction TV & movies, but it's been bothering me lately. Why can ghosts walk through walls, desks, people, pets, but they don't fall through the floor to the ground? Some of them can even sit in chairs when it's dramatically called for! These aren't ghostly wafting specters that float above the ground, they walk on solid ground just like any other character, and take elevators to get to different floors. They ghostly move through everything except the floor.

Beyond the oddly hopeful fact that no character who dies in a science fiction story actually stays dead - it just means future special guest star appearances - why do story producers ignore the ghostly aspects of the character they so gleefully killed off?

This is almost as noticeably plot-driven action as doors on space ships opening and closing in perfect timing to the emotions of the scene being played. The greatest Star Trek invention wasn't the warp drive, or the medical tricorder, it was the emotion-sensing doors. This provides a frightening look at the future if "reality TV" continues on - just when will the doors decide that the emotions are so high in the room that they slam shut on the game contestants they don't like - won't that kill ratings, everyone loves watching the contestants they hate. Maybe that is why those shows are typically filmed on tropical islands that don't have doors, the fear that doors can already sense emotions. They've been doing it for 40 years or so on the TV, maybe people now believe doors actually open and shut in time to the drama being played out around them.

There are many ghost hunting TV shows now, but as far as I know, none of them have captured a ghost on film. So comparing dramatic ghosts to the real thing is practically impossible, and I admit they could actually walk about like living beings. I have no evidence to the contrary. I actually have no evidence that ghosts even exist. But it does distract from a story when an audience thinks "that can't happen". The thought of “that can't happen” overrides the thought “that's a good story”.

Science fiction allows story creators to create non-existent worlds, but if the worlds coincide with the "real" world, they do need to follow some of the physical laws of the known universe. If ghosts can move through anything... then it's anything, floors included! If they are floating above the ground... then they shouldn't walk like normal living humans on the floor.