Loads of characters in art, fantasy, and whatever use metal shields. Runescape is full of them... Link has one... There are a bunch of greatshields in Dark Souls... Captain America throws one... And so on.
I looked on the internet to see if metal shields were ever used in history, in fights, when they would have been "new". I've found a lot of people saying that shields were usually made mostly of hide.
There's a reddit thread of people saying steel shields didn't exist (and not saying too much about why not).
Once in a while someone will claim that metal shields were "rare". So I really want to find some example of an exceptional person who went around fighting battles with a thick metal shield. Or some missing-link explanation why there's no evidence of metal shields. Or an undiscovered treasure trove of 15lb kite shields.
Most of the people I found just say that metal would be too heavy. I guess swords and stuff aren't that heavy.
The National Museum of Denmark page says there were bronze shields but they were only ceremonial and physically extremely weak.
There's a pretty famous guide, or experiment, done by the University of Leicester Archaeological Services, where they built a shield out of bark. Looks fun.
There are some pictures of bucklers which have hooks on them to break swords with.
For now I'm telling myself that shields weren't metal because it's a shock absorption thing.