Mostly a case of perfectly intelligent people throwing wads of money into the market on autopilot because that's all the 401k lets you do. Lambs to the slaughter!
edit: Example: Back when I was on 401k, was into a S&P500 fund that had an expense ratio over 1.5%. It may not sound like much, but that's compounded year-after-year. All of the other funds the 401k allowed had equally scandalous expense ratios.
After starting a business, I rolled over into an IRA. Switched to a different S&P500 fund with expense ratio of 0.09%. Same curve, but I keep more of my money.
It would be nice if people had more latitude in what they could put their retirement into--especially when it comes to tangible things, like buildings or equipment--that you can get some value out of even if the market happens to be down when you retire.