The median US household has ~$12k/year available to save/invest after all ordinary living expenses, per the US government (BLS), and that number goes up very rapidly for people above the median.
Americans are notoriously poor savers, also per the US government, but a large percentage of all households -- at least 40% -- could fairly easily accumulate $1M if they were diligent about saving and investing a decent fraction of that surplus income. The surplus income is available but Americans choose to use that income for things other than saving and investing.
That surplus income is only surplus until you have medical event, catastrophic loss, or need a major repair (roof). Ot to mention the need to save for retirement. It's those extraordinary living expenses that kneecap you.
Whiler I agree with the thrust of your comment, your committing an error: the distribution of incomes isn't uniform across time/age. That is, the set of under 30 and under 40 household with 12k/ year is lower than 40%. If you look at not the median household at this moment, but the lifecycle of the median household from when it started to when, it probably couldn't save 10k/yr until recently.
Americans are notoriously poor savers, also per the US government, but a large percentage of all households -- at least 40% -- could fairly easily accumulate $1M if they were diligent about saving and investing a decent fraction of that surplus income. The surplus income is available but Americans choose to use that income for things other than saving and investing.