Why could Tyrannosaurus rex hands not grasp prey or reach its mouth?

The forelimbs of Tyrannosaurus rex were remarkably small, measuring less than the length of the shoulder blade. While mechanical reconstructions indicate these arms possessed enough muscular strength to lift approximately 180 kg (400 pounds), their limited size and reach prevented them from grasping prey or touching the dinosaur's mouth.1

Paleontologists suggest several evolutionary reasons for these reduced appendages. One theory is that small forelimbs kept the arms out of the way of the animal's massive, snapping jaws during feeding, thereby reducing the risk of accidental injury, amputation, or infection when multiple tyrannosaurs gathered at a carcass. Other researchers propose that the forelimbs may have served alternative functions, such as striking or slashing prey during close-quarters combat, acting as pelvic claspers during mating, or playing a role in social displays.1