[general] When someone gets into some sort of situation where they can pass through objects and people, why don't they fall through the floor?

Whether it be interdimensional shenanigans, or ghosts, or whatever, the floor always seems to be the exception to objects passing through the ethereal protagonist, even if they're on an upper floor with rooms below. And we're not talking people with active control over their solidity, but people who accidentally get into these situations.