If you retire during a recession, your planned 20 years of 401K runway may not last you more than 12. Or you might end up living longer than 20 years. Or both.
With a guaranteed payments pension, you will continue receiving benefits as long as you live. This aspect of it wont matter to you if you fall over dead at 68, but will matter greatly if you end up living longer than planned.
Right now, less than 5% of my retirement savings are going towards a pension (Social security). I would be much happier if that number (And the expected returns from it) were closer to 25% - and that's with the expectation that SS payments will unexpectedly drop in the future. Having an investment instrument that will pay me until I die is great for diversifying my portfolio.
Thanks to that, a fixed benefit pension as part of your retirement strategy is much more fiscally responsible than going all in on either a 401K, or a pension.
* with the current state of medical science.