Something has to be making up the difference between the $20 copay and what it cost the doctor.
I opened an office in a poor neighborhood in Boston in 1982. It didn't take long to figure out that it cost me $26 to do the visit that Medicaid was paying me $8 for.
It took a bit longer to discover that, if I did the same work in the hospital outpatient department, that they paid the hospital $192.
Aha! said the young me. If only Stalin knew!
So, I went downtown with a business proposition. Pay me $46 instead of $8, I keep $20, send me all the $192s, you save $146 every time.
Alas, I missed the key factor. Every penny of the $192 paid to the hospital stayed "in the system". The state could muscle the hospital to hire their friends, start "programs", etc, etc, whereas my $20 would be "profiting from suffering", but more importantly might be used to buy my son a baseball glove or some other socially useless purpose.
They would much rather pay the $192, as it turned out, as long as the most I could get was $8.
The $20 cash doctor visit is a mirage. Too many powerful interests are at stake to allow it to take root.