Well base damage of the bow doesn't change either. Unless you mean the displayed damage in the inventory?
The damage formula for bows is (bow damage + arrow damage) * skill * perks * etc. So the arrows make a lot of difference.
And I don't know about the NPC mages doing more damage than PC mages...
I get that the base damage for bows isnt the listed damage, but the listed damage for arrows never goes up. Are you saying that's a "bug"?

I usually sneak or stay in hiding. I never thought that running towards my death was a good idea anyway. Ghehe. It seems that if I want to play a non-ranged character it is time to invest in stuff that makes me a tank.