The problem is portability. Even with silver that can be a problem. I had to move the contents of my parents' safety deposit box across country. They had 400 oz of silver. I wasn't going to check this as baggage, so I took it as carry-on. 400 troy ounces is 33 1/3 pounds, and basically fills a shoebox. Since it was so heavy, I purchased a tool box to carry it in, and the total weight was about 50 pounds, including the other contents of the safety deposit box.
When I got it to its destination, my parents decided it wasn't worth it to continue paying for a large size safety deposit box to keep it in. They're in assisted living hence they don't have any hiding places at home to store it, so we to took it to a gold and silver dealer to sell it. At $16/oz, it was around US$6400. Yes, a nice chunk of change, but you're not going to want to carry around enough money in silver to, say, buy a car.
Nickel or copper would be even worse, unless you're just going to bury a huge chunk of it in the back yard.
This slide was made of metal.