Given that we have a lot of evidence that trex would swim to islands in the middle of north America to hunt (there was an inland sea here 66 million years ago), I think them having very good long distance vision actually makes a lot of sense. If they were hunting across flat crasslands and looking for food on islands across stretches of water then it would make a lot of sense that they need such good vision.

They don't need to see a mouse from kilometers away, they need to see if that big Grey blob on the Shor of the island 2km across a strait is a washed up icthyosaur, or a triceratops, or just a rock.