> all while incomes have remained stagnant for decades upon decades and educational attainment has ballooned. How does one earn more money while also earning the same amount of money...?

There are two issues that explain that.

First, the stagnation period begins from an artificially high point. A point where US global economic dominance peaked in the ~20-25 years after WW2, because the other dominant powers were mostly busy rebuilding their decimated cities and infrastructure. The US was able to immediately swivel its industrial base to traditional economic output. It pushed the US share of global manufacturing to over 50%. That extraordinary share of global manufacturing output provided a vast, artificial over-abundance of blue collar jobs. That's a ridiculous comparison point to start from and impossible to replicate (short of blowing up the world again).

Second, most of the compensation growth has gone into benefits that aren't counted in the salary figures that show income stagnation. Per capita healthcare costs have increased by ~$10,000 in 35-40 or so years (a six fold increase), and about ~40% of the US gets its healthcare through an employer.

The median full-time wage is $50,000 in the US. The cost to insure a solo 35 or 40 year old earning that median full-time wage, has gone from $40-$60 per month in the early 1980s, to more like $500-$600 today at a minimum. If you're earning $80,000 as an engineer for example, your insurance compensation might cost the company more like $800 to $1,000 per month.

A family of four - two full-time employed adults - earning $150,000 per year can easily be spending $25k-$30k per year on health insurance.

If you have two people earning $90,000 together, with one child, their insurance cost is going to be $1,500 per month or more - $18,000 per year - representing the equivalent of 20% of their salary. All of our wage growth potential is going into that blackhole of healthcare cost expansion.

The UK, Japan, Australia, Canada, France (elite healthcare systems globally) are all spending $4,000 to $5,000 per capita per year on healthcare costs. The US is spending more like $11,000 to $12,000 ($3.6 to $3.8 trillion per year roughly). The US share of its GDP going to healthcare costs, has about tripled in 40 years, from ~6-7% to ~18-19% today.

That's where most of the wage gains have gone, to a $1.4 trillion per year giant wealth transfer to the people working in all the layers of healthcare. The wage gains have been diverted to subsidize the existence of jobs and wages in healthcare that otherwise would not exist. Admin, doctors, nurses, insurance salespersons, pharma reps, scientists, shareholders, buildings, janitors, and the zillions of other jobs and costs (artificially high wages) in that system that otherwise wouldn't exist if you adjusted the US per capita spending to match other developed nations.

Did you know the US has 3x to 4x (43 CT and 38 MRI units per million people) the number of CT and MRI systems per capita that Canada has (15 CT and 10 MRI units per million people), and about 2x to 3x the number that France has? An example of wage diversion to feed the healthcare monster. That provides jobs that shouldn't exist for healthcare professionals, jobs for repair & maintenance people, jobs for sales people, profit/revenue for GE & Toshiba and so on.