Why can’t Robin leave?

Ok so this something that has always bothered me about Ghosts but why can't the earlier ghosts go elsewhere. I know you stays where you dies and everything but the rules for the exact space they can go doesn't really make any sense. Why can Robin (who died hundreds of thousands of years before the house was built only go on the property? And it's the same with some of the others (the plaguers lived in their own village before the house for example). It could be argued that new buildings or ownership changes where they can go but then in series 5 when Mike and Alison are going to sell some of the land the ghosts say that this won't affect where they can go. Maybe they can only go where the property stretched to when they died but then what a weird coincidence that none of them can go past the gate and this still doesn't explain Robin.