Consider this... perhaps the weapons we find are already "improved" by the NPC smiths to the best of their ability. Anything you do to it after you find it is beyond them.
No no no no no no no, that's not a valid argument.
Alvor in Riverwood can teach you how to improve a Iron Dagger and a Hide Helmet to (fine), he knows that they can be improved and how to improve them.
And Alvor is just some random blacksmith in a small village, That guy up at the Skyforge is suppose to be "legendary".
That he simply cannot
ever raise
anything up to (fine) and beyond breaks the logic of the game.
Or Alvor is the true legendary blacksmith.