Why?
Because races are very diverse in TES,or are supposed to be.
Orcs have been trained since child birth to learn about heavy armor,its in ther blood.
Like wise with Bosmer,they have beeb trained there whole lives in archery and stealth.
Should an orc that has been trained his/her entire life be suddenly able to learn how to sneak and use a bow.
I suppose with a good backstory this could easily be done.
But for the majority of Orcs this would not work.
I'm going way off topic here but I think there are other ways of adding to the just +5 heavy armor racial bonuses.
My thinking is that they could put a cap on certain skills or better yet just slow the level progression of certain skills.This would allow for good starting skills to be more usefull in the game.
But you are right it makes no difference in this game.