Lots of U.S. wealth is in equity (stock exchange et al) I believe.
Hence 401K is so much important there.
P.S.
This in turn I believe results in some contradiction.
So 401k performance is a good metric for politicians during elections, therefore policies are geared towards propping the stock exchange, see Trump emphasizes the Stock exchange.
Yet, 401K values are disproportionately tilted toward old workers/pensioners, being a pension program it is. Ideally, government policy should also take account of young workers/voters who are recently starting to build their economic nest and wealth. However, young voters are kind of unreliable i.e. low turnout so politicians rarely cater to their needs.
Harmful effects of overemphasizing equity/401k are for example expensive real estate and/or companies tend to over-optimize their quarterly financial reports, sacrificing workers' career and salary progression.
Disclosure: Not an economist, just a speculation.