I'd imagine that gold is used for the purposes already mentioned.
As to other metals, I'd imagine that there are mines and other places to produce them, though they're either imported or simply not shown in game.
Also notice how all the Mines in Oblivion are abandoned, crumbled, shattered, etc?
Along with Ayleid RUINS, Ruined Forts, Everything is ruined.
Well, with the mine you have to clear of goblins in one early Fighter's Guild quest, it's at least implied that it was only recently abandoned due to goblin trouble.
But in any case, you're right. And that really doesn't make any sense from a lore standpoint (except Ayleid ruins, as the Ayleids aren't in power in Cyrodiil anymore.) I think the thing is Oblivion wasn't trying to make sense in that aspect, Bethesda decided that they needed more types of dungeons, and asked themselves "What would fit the game we're making?" and came up with fort ruins and abandoned mines, then proceeded to fill Cyrodiil with them, but never thought to put a few non-ruined/abandoned ones in the game for the sake of realism.