Another is that monetary policy has made fixed income securities a low yield investment. Funds need to diversify and add risk.
Another reason is that folks writing these things assume workers work a full career. The reality today is that many government workers start in their 30s and 40s, and only get the rich benefits if they work until their late 60s. That neutralizes the problem, as they will be dead sooner.
Personally, I think the WSJ and other anti-pension people overplay the liability. The drivers of pension expenses are police and firefighters, and these (mostly) guys tend to die young.
They also tend to report on the problem in masse, while it varies from outright disaster (Illinois) to Oregon, which is overfunded.