That's a misleading way of comparing them because the doctor has radically higher startup costs in both time and money.

Once that's factored in, the doctor still has higher lifetime earnings, but not nearly the 4x you're pointing at. More like 1.8x.

And it's not until about age 40 that the doctor breaks even relative to the plumber.

And the doctor can only pull all that off in the first place with access to about $500,000 in credit.