Tyrannosaurus rex’s short arms, why?

This 26 June 2018 video says about itself:

Tyrannosaurus rex was big, Tyrannosaurus rex was vicious, and Tyrannosaurus rex had tiny arms. The story of how T-Rex lost its arms is, itself, pretty simple. But the story of why it kept those little limbs, and how it used them? Well, that’s a little more complicated.