Because wizards read books and don't hit the gym and warriors hit the gym and lean into brute strenght .
And likely most wizards will have low strenght and won’t invest in athletic skills. Sill, if one want to make a more interesting build with multiclassing, I see no reason to create artificial limits.
Rule 101 of an RPG - allow players to create a character they want to create. If it’s muscled wizard then it would be nice if game had a use and benefit for that.
Also I remember Swen mentioning that unusually powerful jumps and pushes are tied to tadpole in our head. So there is that - you wizard is being changed into Captain Amer.., Mindflayer.
