Probably something to do with strain on the body. Studies show T. rex couldn't even run. Maybe a fast walk. Additionally, the way the muscles attach to the bones probably don't support hopping.
I was one of those "gifted children that was too lazy" and my mother was one of those "nothing can be wrong with my child or it reflects poorly on me!" and I got diagnosed with full blown ADHD at 29.
Therapy and drugs are great. And I'm glad you were aware enough to get your child tested. Early is for their best.
And it's funny, I'm pretty sure he has a couple accounts. Any time I post something about him, as soon as he responds, I get multiple down votes at the same time.
At a certain width we need to start using the rocket equation.