Compounding interest...if you start early enough and take advantage of employer contribution matching, etc. But most people in the US won't retire as millionaires, let alone even retire. Unforeseen life events often cause people to dip into retirement savings and in some cases wipe out any gains. Also, once in retirement, life in the US can still be quite expensive. For example, Medicare and all the additional supplemental insurance(s) you need is absurdly expensive for retirees. Everything in the US is "out of pocket".

Because the lack of effective pensions require you to either work til you die or live off Social Security. Most people don't want to do either, so they direct a significant amount of their earnings to 401ks and the stock market.

The fact that young people struggle really doesn't have much to do with this.

Old people can’t work. They’d better be rich enough to sustain their lifestyles on investment income, otherwise they’d just die.

The bug is working people struggling not middle class people having livable retirements