What percentage of US citizens even have passports?

All of them. If you do not have internal papers you will be detained
at an airport. BRBR

That is completely false. Please stick to botany.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)