I'm guessing that the devs created iron ore/ore deposits early on with the assumption that all the common metals would be discoverable in their rawest form. Either that or they plan to add more ways to discover iron at a later point, that maybe wouldn't require returning to the swamps. Right now neither iron ore or iron ore deposits will spawn naturally:
https://valheim.fandom.com/wiki/Iron_ore
https://valheim.fandom.com/wiki/Iron_vein

Originally posted by GrouchyMike:

I believe there are iron ore deposits in the swamp, but finding them without an item that helps you find silver is pretty hard.

You are mostly right except it is muddy scrap piles, not iron ore deposits that can be found hidden in the swamp.

Another way of also finding scrap iron without crypts is by killing oozers (big brown blobs, not the little green blobs), but I wouldn't blame anyone for forgetting or not knowing this because I assume most would not consider them worth the trouble of killing.