1. Maintain your own weapons? You mean as in "be the best smith in all of Skyrim, and the only one who can improve anything at all"? Something like that?
2. Because they're not smithed weapons. They are temporarily summoned daedra in the form of weapons. And you don't get weaker; the weapons are simply what they are. By the time you get to high level Conjuration, you can summon twin atronachs or permanent thralls; why do you need a high-level bound sword?
3. Yeah, the crafting is way too overpowered. The system, as it exists, practically forces the player to exploit a crafting skill. And, in the process, unique artifacts are trivialized. My Skyrim characters leave high-level enchanted loot in boss chests, because what they made themselves is better, and the stuff is too heavy to bother lugging it out to sell.