I assume the reason Piglet in particular doesn’t leave the estate is because she’s never been trained and has no recall whatsoever (the other two don’t seem much better to me but clearly they aren’t so poorly behaved as they get taken on holidays).
I’m sure the very modest 120 acres (haha!) provides lots of rabbits to chase and badgers to hunt (and I wouldn’t say no to that kind of land for my dog!) but I do think it’s a bit sad Piggy (and the others?) don’t get the sniffs and smells of others dogs, cats, people etc on more ‘normal walks’, which I think are mentally stimulating and tiring for dogs.