I don't know why this would be unbelievable or "whopping." $3.3M over 60 years of adult life (age 20 to 80) is $55k/year. That's pretty close to the median American income.
And the median household income is actually $75k now. I'm just saying that $55k is very much in the ballpark for the median person. Does it really matter if that's before or after taxes?
Agreed, but there was no room for savings in your model as you used the whole income as spending (median income * 60 years). Taxes makes a big difference too since it doesn't count as spending. E.g. someone that earns 55K$, pays 25K$ in taxes and saves 10K$, only spends 20K$ per year, amounting to 1.2M$ (20K$ * 60 years).
For a family in 2024, at the Federal level, the first $29,200 is untaxed because of the standard deduction. The remainder would be taxed at 10-12%. Child tax credits can easily wipe that out and make the liability negative (i.e. you get all your tax withholdings refunded plus another couple thousand dollars).
State income taxes will vary, but are typically lower than Federal income taxes. Many have no income tax.
"401k" is obviously not an "expense" independent of the things that 401k savings are actually spent on (I mean, unless one is counting the expense ratio on their mutual funds).