They seem to overlook the opportunities that volatility affords. If you buy at the right time (ie when prices are low) and sell near the top of the next boom (yes, it's going to happen, just a matter of when!), then you will do well.
You may not do so well if you buy in a more stable market where the gap between current and future value is a lot less. (granted, it may be better for peace of mind.)
Unfortunately most investors tend to buy too late in the down cycle and sell too early in the upturn. That is why we are not all millionnaires.
Some people also think they can predict the future, which a risky supposition. My crystal ball says: "Ask me what happened yesterday. As for tomorrow, ask me then."