The answer that covers many cases is: they otherwise have no retirement.

When I worked for a state university, in IT as a senior DBA, I earned half what private sector friends and professional acquaintances earned.

The story was something like "well, we can't afford to pay you enough to save for yourself, but we'll make sure there's a decent retirement to draw from for you."

The problem is, after the housing bubble, and with low-interest rates, the investment methods they had relied on haven't kept up.

Not saying this covers the more extravagant pay outs. But public workers on the whole have largely been shorted up front in exchange for a decent retirement.