It isn't that governments are bad at it, its that they have incentives to fuck it up. It's much easier politically to give the union what it wants and let it be some mess someone cleans up in 20 or 40 years.

Only if the electorate is intelligent / educated enough to call bullshit do they temper their bullshitery.

The electorate prefers to be distracted by Buzz Aldrin's facial ticks, Pence's water bottle, and a fake Meliana blow up doll instead of having a serious discussion about the consequences of policy today, let alone 20 years from now.

(Just three recent examples of the nonsense of the day. Yesterday or tomorrow, different nonsense, but always nonsense over serious discussion.)

Probably some governments are bad at it because knowing what will happen in the future, which you need to in order to keep a defined benefit plan solvent, is impossible. Other governments/employers were luckier.

But it's not so hard to say that you can do it.