The problem is not just the material you use, but where you keep it. Most people don't have a safe at home (and even if they do, it's not 100% secure). Metal or not, your physical seed can be lost, stolen, burnt...
Most people can remember many the names of persons (2 or 3 complex words) for decades. I don't see why they could not remember 3 or 4 simple dictionary words.
A safe is an obvious target, why is it considered safe to place it in a safe? Everyone has their own method of storing items, and usually adding a passphrase is much safer.
The human brain has a hard time remembering random things, even just twelve random words.Even a random eight-letter password is impossible for most people to remember.
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