A 401k is an individual investment portfolio for retirement.

You put in some money every month. Your employer may or may not match you, to varying extents. The money in the portfolio grows with investments over time. When you're young, it uses higher risk higher yield investments. As you get older, it shifts to lower risk lower return investments.

Once you retire with a 401k, you basically retire with a limited sum of money on which to live out the remainder of your life. If you didn't put in enough money into the 401k, you're fucked. If some economic crisis struck and your 401k investments tanked, you're fucked. Basically your investment portfolio has to have been managed really well over the course of your life for you to have enough to retire on.

A pension plan doesn't work like that. Pensions are not individual plans. They're collective plans. You pay into the pension, and your money goes into the same pension fund as every other member's payments. When you retire, that collective fund pays you a fixed monthly pension no matter how long you live. You never have to deal with an individual cap on your total benefits.

Furthermore, pension funds are considerably more resilient to economic catastrophes as well. If the fund's investments are hurt by a crash, the fund can effectively "borrow from the future" to support the pensions that need to be paid right now, and then build the fund back up at a future date when the economy is thriving. In essence, pension funds operate across generations and so they can leverage economic prosperity phases to make up for depressions/recessions.