Except that 401ks are invested. By default they tend to be invested in a relatively stable, diverse portfolio along the standard long-term investment guidelines of ~60/40 balance of stocks and fixed or cash holdings. Mine made 15% last year invested even more conservatively than that, and it's a no-name 401k provides by my small employer. I would have made significantly less with gold.
If you think people's 401ks are just sitting there in a low-interest checking account, I don't think you understand how they're actually structured.