Because wizards read books and don't hit the gym and warriors hit the gym and lean into brute strenght .
( Edwin never had the makings of a varsity athlete )
Rule 101 of the rpg -
But - the enemy takes the high ground and you have all the tools in the first turn to negate his said advantage. Becomes pointles if an ambush on high ground is 100% a ticket down . AI is already dumb, having him take extra hits is not helping .
Maybe in higher dificulties you would take some caution on finding enemies .
( than again i refer to like 3 gameplay videos on YT where the enemies were there just to take the beatings - )
Some penality on str , mobility , agility and battlefield coverage should be given to different classes ....even there is non in 5E .
And 5E is just an number , so that means sometime in future there will be a 5F or a 6A ..... you can always improve on rules sets as done before too
