Cannabis isn't "mind-numbing", it activates the brain and helps repair brain cells. I think you are thinking of alcohol.
It can be physically relaxing, maybe you are confusing mind-numbing with relaxing. Although plenty of top athletes find that it helps with physical activities as well.
The Diaz brothers, for example, smoke a ton of herb before they go out and train for triathlons.
Ah, my mistake. Anyone and everyone can handle copious amounts and regular use of weed and suffer no negative consequences.