The Hedge

[Lalwendë]

Why a hedge? Why, the answer's simple!

The English love nothing more than growing a ruddy big hedge right around their property. Especially so if there are neighbours there and there is the potential to block off all their natural daylight. I suspect that the Hobbits would have made use of that scourge of England, the Leylandii tree, one tree that I would more than welcome Saruman's hordes to chop burn and destroy. Hobbits are certainly insular enough to indulge in this heinous pastime, so I envisage the Hedge as a row of fifty foot high, funereal, evil Leylandii.

[Selmo]

The hedge was long (20 miles?). A vast ammount of stone or brick would be neded to replace it with a wall.
While I accept Lalwende's explanation as highly likely, perhaps another reason lies in the Hobbits' persistance in living in artificial caves long after they had migrated from real caves of the mountains; there was a shortage of good building stone in The Shire.
.