[dahdum]

They are more similar to commodities like gold, silver, or other precious metals in that they behave much like monetary assets.

Tons of people do invest in gold/silver for retirement.

[Xixi]

Gold/silver are also speculative assets, not investments (per Warren Buffet definition, which I like and use myself). Just because they are speculative assets doesn't mean it's a bad idea to buy them.

An investment would be putting your retirement money in a startup for instance. And that would be a very very risky investment with near guarantee of complete loss of capital.