What I wonder though is if the passage is clearly accessible, why is it one way? In essence, why don't the sharpteeth use the passage as well into the Great Valley?
Sharpteeth aren't especially known for their intelligence, nor for their keen awareness of their surroundings. Perhaps word has not yet gotten around to the sharpteeth about the Great Valley.
The vacancy of that stripe of land could be for that very reason, that the dinosaurs of the Great Valley mean to create a "buffer area" where there is no impenetrable Great Wall to protect the Valley.
I think Malte is onto something, regarding the "no man's land" seperating the Great valley from other more fertile land. The apparent lack of food between where sharpteeth tend to "graze" and the Great valley is little incentive for them to explore the area.
one would expect at least a small influx of sharpteeth into the Valley via this "frontier".
Sharpteeth have been to the valley, based on historical events that Grandpa longneck told in the Lone Dinosaur story from LBT 6.
Unless earthquakes since then have changed the landscape since then, to the point where sharpteeth can less easily find/bother to locate the entrance.
We do know that sharpteeth can talk to each other (from LBT 5) but perhaps they have no real incentive to show other sharpteeth where to find food. They would simply end up fighting over it. Any sharptooth who has been within scent of the Great valley wouldn't in his narrow-mindedness, tell rival sharpteeth about this land of much food.
Perhaps I don't give sharpteeth, at least the full-sized sharpteeth, enough credit, but it just seems to me that they don't tend to look out for each other the same way that the plant eaters do.
We obviously cannot group all sharpteeth together.
Raptors tend to be much more willing to work as a group, though relatively little is known about their bahaviour when food isn't immediately around. We have only seen raptors (LBT 3, LBT 7) when they are within sight of their prey.
The larger sharpteeth tend to hunt in smaller groups, or even solo. The only exception to this has been LBT 10, when 3 sharpteeth attacked the longnecks. If you want to count LBT 6 where 2 sharpteeth attacked, I suppose we could, but there was little evidence that they were hunting together. It was probably just bad timing. The LBT 10 sharpteeth seemed a little more coordinated (approaching very shortly after one-another, from all different angles). Of course, it could have been poor timing as well, but it seems likely that the attack was planned this way.
This is evidence of some sort of intelligence in the sharpteeth, however their fighting ability has degraded a lot since the original movie. The three sharpteeth from LBT 10 seems less threatening than the single sharptooth from the original movie. Even the fighting skills of a sharptooth from the later sequels seem weak and clumsy.
My point here, is that sharpteeth tend to be ignorant of their environment. They can communicate in aggressive situations, but they don't tend to share where to find food. Perhaps food is relatively scarce in many places, and theey worry for their own survival. Big sharpteeth tend to work as solo, or small group hunters. The result is that information doesn't get passed from group to group, or sharptooth to sharptooth. Any sharptooth who has seen the great valley has either not lived to tell about it (LBT 1, LBT 3, LBT 6, LBT 8 possibly - He was practically within smelling distance of the Great Valley), or has returned home (LBT 2, LBT 9).
Also a factor perhaps, is that some sharpteeth know the great valley exists, but they simply can't find the entrances. In LBT 6, the sharpteeth across the ditch from the Great Valley couldn't have crossed like the gang did (over the rock pillers, or over the log). They sharpteeth are likely too heavy and un-coordinated.
There is no evidence that smaller sharpteeth like raptors live in that region. Larger sharpteeth either didn't have the patience, desire, and/or intelligence to ty to go Around the ditch to get to the valley. Perhaps they simply didn't think to try to get there in the first place. After all, there are adult sharpteeth. They have survived on the food they have been getting, for a long time.