While a direct assault would be rather foolish, without inside help anyway, Sharpteeth setting up around the Great Valley to attack Leaf Eaters going into the Great Valley actually makes a lot of sense. Much like a predator would hope to ambush herbivores at a coveted watering hole, Sharpteeth would be well-suited to congregate near the Great Valley to try and ambush Far Walkers heading to the Great Valley.
This is further logical by the fact that some have tried: namely Chomper’s parents (who had Chomper’s egg near the Great Wall but later decided to move elsewhere after being chased off by the Valleydweller, though why is a mystery, as the Great Valley resident often seem too concerned only about what goes on INSIDE the Great Valley to care too much about what Sharpteeth are doing outside of it.) and the Fast Biter mother from the TV Series.
This is even further hinted at still in the third movie when Grandpa Longneck stated that the coveted water in the Mysterious Beyond during the drought was certain to have Sharpteeth lurking near it (which, of course, there were.)