By your math, the US millionaire couple about to retire is actually 'worth' 3 million.
Also, your numbers are misleading. The average lifetime payout numbers include Medicare benefits in addition to Social Security. You should also use estimated present-day values instead of lifetime payouts if you want to compare benefits to savings.
Median full-time income in the US in about $55k. A quick estimate for Social Security payments for someone retiring today at the nominal age with that income is $1580/month. That becomes ~$38k/year for a couple, which is ~$750k using the 5% rule. Median household net worth, including primary residence, is ~$250k at retirement age.
My estimate was based on a median income of ~$40k and pensions that pay ~60% of income. That would become almost $1M for a couple using the 5% rule, in addition to ~$200k household net worth at retirement age.
By these very rough estimates, the median recent retiree is worth 9 years' income in the US and 15 years' income in Finland. That supports the idea that there is a shortage of millionaires in the US.
Yes, that's the problem your parent comment is pointing out.
You're not trying to count them either. The value of a pension isn't the payouts it provides. It's what an identical financial structure would cost. A pension that pays out $1 million over 15 years is worth less than $1 million.
You appear to be correct on the original point that US social security payments are pretty comparable to Finnish pension payments. As of March this year, the average Finnish payment was 1784 euros and the average American payment was 1537 dollars.
In a Western country with mandatory pensions (such as Finland), a mid-income couple should have effective net worth over $1 million (excluding primary home) by the time they retire. They are just not counted as millionaires, because they have pensions instead of retirement savings. Had they been able to save for retirement instead of making pension contributions (which are mostly used to fund current pensions), their net worth would be twice as high.
A recent middle class retiree should be a millionaire. If most of them are not, there is probably something structural that prevents people from saving as much as they should.