Who knows really what the imagineers had in mind. Maybe it is just a retirement home and this happens all the time in a cycle that is never broken and there is a new caretaker on the job who has never seen this before? Too much to think about for me...I find it more appealing to think that they do inhabit the house all the time but this "getting stuck and materializing" business is a one time deal that we are the poor caretaker are lucky enough to see.
Also if this happens all the time why would the hitchhikers always need to fill the quota? Wouldn't you think by now someone would have joined them? And the mansion would have run out of hitchhikers by now because one goes home with every person and they will haunt you until you return. Well some people never return?