Have you ever attempted to fix a broken hammer with a wooden handle. Doesn't work. Get a steel one. Some things can't be repaired.
Sooo.. First of all, to actually FIND a good (Heavy-/X-)Bow/Staff is a terrible journey itself. Most weapon types you find are swords...then fencing style weapons and then....way behind good (two-handed) maces... Archery follows on the very last place, far behind maces... And IF you get a good (mace 2-handed) / archer weapon, it's impossible to repair without spending 10k gold for a powder of fortifying? Just sucks. On all the other shard, including OSI, it was possible to repair those weapons. I remember handing out my weapons of that kind to a GM carpenter/fletcher i could trust to repair them, even without repair deeds.
Does seem perverse to me that they can't be repaired. Don't make them any stronger than they are, just make it so they can be made to last a little longer. Haven't seen a convincing argument against that.