If biting was the main method of fighting for the T-Rex, they must have relied on their large hind legs to run fast, fast enough to get no more that a Crocodile’s Snap of the jaw away from their prey.
I have read that skull construction indicates that T Rex could smell rotting carcasses 20 miles distant. Some modern bears have the same ability. Fast legs would get him there before others found the food. Arms would not be very necessary to eat a stationary meal, and big jaws would scare or bite competing preditors and also allow big bites at a time. In other words his physical traits were more oriented to finding, eating and protecting food, not hunting and killing it.