There are two reasons you might not want all retirement savings money to be wholly in control of the saver:
1) Some folks are simply really bad at saving, which ends up being rough for others around them and for society, not only affecting them. This reason tends to rub some folks the wrong way on principle, so they may prefer to disregard it, but it is true as far as it goes (principle aside) [edit: I mean doing anything about this for this reason rubs some folks the wrong way, not that they disagree it’s a real phenomenon]
2) Money directly available to people is freed up for rivalrous zero-sum spending. Think: bidding up scarce resources for your kids, like good schools (which can mean housing). In a world where 100% of comp is employee-directed, this punishes responsible savers.
Edit: Although it does appear there is a cap to the employer's contribution ($69,000 for 2024 [1]). But I think the general point still stands, why bother to have employer and employee limits.
[1]: https://www.irs.gov/retirement-plans/plan-participant-employ...
A $200k/yr employee with no employer contribution would be limited to $23,500 contribution (in 2025 limits).
[edit] actually that’s not quite true, though, because IIRC contribution rules have to be uniform, to avoid horse-shit like maxing out upper management at $70k and contributing nothing for lower-level employees, limiting them to $23.5k tax-advantaged no matter how much hard try to save, I.e. to prevent the whole damn scheme from benefiting mostly the already well-off more than it’s probably going to regardless.
They'll pay it when they take it back out. At best they're saving the difference between the bracket rates in exchange for letting your money slosh around the markets for your working life.
500k in income 1 year is ~$162k in income tax.
500k in income split across 2 years is $68.5k each year totalling $137k.
It's not massive but it's still a decent chunk of taxes people can avoid.