Well just like yer garden fence posts: D'y'see, they can go rotten at the base. The changin' seasons, exposure to the elements, poor maintenance, wind damage. etc.
The list goes on and on.
Some are just plain fortunate. There's no other way of puttin' it!
But some just 'ave to be dug up and replaced. It's the only way.
Bingo! New fence! New boundary! No more neighbours from ....you know where
Mr Rosscadaver
Pat(for now)