Dil likely didn't try to get in cause maybe she doesn't know about it. Plus to get there, she has to cross a vast desert and that isn't good for her kind. And of course, she'd need Ichy, whom she knocked away at the end of the 4th film, and seems to hint the end of their partnership.
As for sharpbeak flyers, true, they could get in. However, there are plenty of flyers in the valley that could ward them off. Plus sharpbeaks probably would only try going after children, really young dinosaurs, and there are plenty of adults around to keep them away. Sharpbeaks are also more fragile and light, meaning a blow to them is more devastating than with a terrestrial animal, so they would be a lot more cautious.
In the end, it's possible their reason for not coming into the valley isn't so much that they can't; it's that to them, it's probably not worth it, considering the valley's reputation.