Because ‘metal’ covers such a wide range of materials? Aluminum is a metal, and so is magnesium, lead, silver, cobalt, tungsten, and mercury.
Again, most folks aren’t particularly solid on what constitutes the molecular differences between iron and steel, so the character sees it and thinks ‘iron’, while the player can easily read in its materials that it’s made of steel. This gives us the best of both worlds; the in-character description as well as the out-of-character information contained in the item’s information block.
