Because the collective nature of the investments in a defined benefit system allow time for market recovery both before retirement and during the retirement years, and an individual invested soley in a 401 (k) has no such chance. But I suspect you knew that.

If the only factor involved is time for a market recovery, as your response implies, then the pension fund should be expected to recover without any increased contributions from outside the system -- so can we tell the people of California that the "employer contriubtion to state pensions" item in their budget won't be growing, as a result of the market crash?

Sadly, no, because while employee contributions to public sector plans are fixed (more or less), the government employer funds (hopefully) to the actuarial minimum, leaving no margin for error, so increases are needed based on the new data. But in most cases, the biggest gains will indeed come from the market recovery, not the actual contributions. In a perfect world, the contributions would be x % higher than an expected minimum over set periods of time, so that such fluctuations could be offset over time, but that would be trading extra funding up front for predictability, and politicians run based on short-term, not long-term, outcomes.