> A 401K is just an investment in indexes linked to your pension, there's nothing stopping everyone putting their own money into an index themselves

This is mostly incorrect.

A 401k in the United States is a tax-advantaged investment account. You can buy shares of individual companies, you can purchase index funds, you can leave it there as cash, buy gold, or just about anything you want or you'd expect to be able to purchase using a brokerage account. Depending on the route you take (Roth or Traditional) you can realize tax savings now or at a later point. 401k Accounts are programs offered by your employer as well [1].

One of the many reasons the OP's question doesn't make sense is because not every country in the world has a 401k program.

[1] There are other programs for individual owners or for those who have an employer that does not offer a 401k.

People can invest in markets without a 401k with more options (plans commonly have only a handful of funds available) and less fees (both admin fees and inflated fund expense ratios). And you may pay more taxes with a 401k than otherwise depending on your future tax rate (which is unknowable).

The only pure advantage is employer matching if you have it and stay employed long enough for it to vest.

So not exactly a necessity to build wealth.