I guess the reason is, is that from a design perspective, there are a relative few number of creations and improvements done through smithing than through the other two crafting skills, which means that it would make the smithing tree far less useful (as opposed to alchemy and enchantments where finding the right combinations are harder).
Still, it is a tad bit odd that you can buy something from a great smith, and assuming you had the materials, could always make at least a marginal improvement (to fine) with no skill in smithing.
One thing I think that would have this make more sense is if smithing bonuses wore out with use and had to be reapplied (note, this would not wear down weapons below the base level). This would also give you reason to keep getting smithing materials even when you have end game armor and weapons. For alchemy you need to keep locating ingredients for your potions, and for enchanting, you need to get more soul gems (though the star does mitigate this) to recharge your weapons. As it stands now, once you have your final armor and weapons for you and your companions, it's still a good idea to locate materials to keep them up to date.
This would also have the side effect of allowing you to find improved items in the world, and still keep smithing from being useless.
Well, my thoughts anyway.
Edit:
Another thing that this improvement would do is increase the usefulness of the Arcane Smithing perk. Right now, if you were an enchanter, you could improve the base weapon and then enchant it, strongly decreasing the usefulness of the arcane smithing perk. If improvements were to degrade, the only way you could reimprove your enchanted weapons would be to have the arcane smithing perk.