In movies, how come ghosts cannot touch physical objects and can pass though door/wall but at the same time able to sit on couch just fine?

Hello,

Last night I was watching a movie called Just like Heaven 2005 and I just had to ask this question. It has puzzled me for years, is there a ghost rule for this thing or it’s just a weird way to make movies bearable. Or is it just a plot hole maybe? They usually never explain this phenomena.

How come ghosts are able to sit around in the car or in couch but unable to touch objects at the same time. Please explain this or point me to a webpage explaining the same.

Regards!