I've only seen that on SS website calculators - and even then you can put in a bunch of zeros for the years (decades?) you don't work or contribute into SS to get a more accurate number. ...
What you've seen is then quite different from most retirement calculators I've seen aimed at the mass market, perhaps partly because it's a government pension. Just running the numbers on the assumed plan value using my own employer's (private 401k, as others had discussed upthread) plan calculator makes it very clear that it anticipates employees will
1. not retire until, at minimum, age 50
2. continue contributing at the same percentage of the same income until that date, with no option to add years of non-contribution between the present and age 50 (no exploring other jobs, let alone take time off to raise kids, take care of elders, travel, or be unwell; mush, doggies!! grind until you drop!!)
and, aside from this
3. that you would retire to withdraw each year a percentage of your current income not less than 50% (recently amended to allow dollar amounts, an unexpected step in the right direction!)
... Another reason is the employers who hire these companies might not appreciate them revealing to senior, top-producing employees how they could retire next year by maxing out their 401k instead of spending the next 5 years working and contributing 5%.
Oh, man, I truly wish I didn't find this so plausible. They are on the employer's payroll, after all.