I don't know if this counts as 'purely historic' or not, but I think the pragmatic answer is simply that it's the law. The land was bought by its present owner

I don't know if this counts as 'purely historic' or not, but I think the pragmatic answer is simply that it's the law. The land was bought by its present owner knowing that the public had a right of way across it. The law could change, but there's no particular reason for it to any more than there is to argue for it to exist from first principles.