401k's are one of the primary methods of defined contribution plans, although many companies and government agencies have their own defined contribution plans. 60% of workers have one of these plans (less than 15% of workers have a defined benefit plan that does not depend on the stock market to define their retirement benefits.)
Defined contribution plans are very heavily invested in the stock market. The growth or decline of the stock market has a huge impact on the retirement benefits for 60% of workers.
http://www.bc.edu/centers/crr/facts/jtf_13.pdf