Hmm...that is something to consider. I don't know about using false walls; I just assumed the routes in and out of the Valley were fairly difficult to use - consider the high route taken to get to the Valley containing Saurus Rock, or the rocky mountain route used during the heavy Winter.
Someone else suggested that the Sharpteeth normally avoided the Valley cause there were too many Leaf-Eaters - enough to fight back. This works for some of the Sharpteeth seen - Chomper's parents, for instance, who were only entering the Valley to find their kid - but falls apart when you remember the Sharpteeth who snuck into the Valley after the Tinysauruses.
Alternatively, it could in fact be a clever counter-scheme by the Sharpteeth. They don't enter the Valley because they want there to be a stable supply of Leaf-Eaters living in there. Instead, Sharpteeth linger in the areas around the Valley, and pick off the odd Leaf-Eater heading in through one of the smaller routes. Wait...that doesn't work; the Original Sharptooth was working to get inside, and so was Redclaw, I believe...perhaps they do that anyway, but simply haven't found the routes in?
This needs greater examination...