My theory has always been that the sharpteeth simply don’t want to enter The Great Valley. They are smart enough to know that they would have an easier time getting a meal in The Mysterious Beyond then having to enter The Great Valley.
It’s a simple fact that The Great Valley has herds of different species that work together. No single sharptooth can hope to get a meal in that situation. However, with The Mysterious Beyond, a sharptooth has a much better opportunity. They can easily get any stragglers or find a loner and have them for lunch.
This has always been my theory. Sharpteeth are just simply smart enough to not enter The Great Valley in the first place. They’re better off that way.