... benefits the inhabitants of the great valley. It appears that only a very few entrances allow for the passage of large dinosaurs (such as the entrence in the fourth movie.) It is also possible that the "hunting ranges" of the carnivores ("Sharpteeth") do not extend to the passages through the mountains into the Great Valley.
Furthermore, it appears that the odds that a carnivore could enter the Great Valley are remote, but it is still a possibility. It also looks like smaller creatures can enter with no problem, even malevolent ones (such as Ozzy and Srut in the second movie.) Another factor that comes to mind is the harsh terrain surrounding the Great Valley. Maybe the predators don't have the determination to cross into the Great Valley, unless they were motavated by some higher purpose (such as when Chomper's parent's recovered him in the second movie.)
It would make no sense for a lone predator, or even a whole herd of predators to enter the Great Valley for food, as they would be outnumbered by the Valley's inhabitants, and would surely meet defeat in a pitched battle, being so outnumbered. Of course, if the predator's prescence goes undetected, they might just be able to live in the Valley. Of course, it would have to be a smaller predator.
