> Isn't social security a thing?

Social Security alone will, at best, slightly mitigate poverty. 401Ks are generally employee-funded, with some firms providing matching funds, especially during good economic times and where the firm is in a field where the main area of labor relied on relatively scarce so that there is competition for talent.

EDIT: The line about social security is a little inaccurate in the extreme case; its actually technically possible to reach a moderate income ($62k/year) on Social Security, if you have a long enough working career (35 years or more) earning at the maximum taxed wages for Social Security (currently $185k+) and claim at or beyond the age that maximizes the benefit calculation (70 years).