> Why can't you go to a retail store, buy an anonymous prepaid debit card for cash and then use it to make purchases over the internet
Because when this is made available in practice like 99% of the volume is tied to crime. 99% of users aren't criminals. But criminals use it at scale in a way that dwarfs licit activity. (Without tracking how much each person is using it, you can't cut off the scale users.)
This is just assuming the conclusion. Anonymous purchases should be the default, so that ordinary people aren't baselessly subject to mass surveillance. At which point you'd have to be arguing that 99% of all purchases are criminal, which seems implausible.
Because when this is made available in practice like 99% of the volume is tied to crime. 99% of users aren't criminals. But criminals use it at scale in a way that dwarfs licit activity. (Without tracking how much each person is using it, you can't cut off the scale users.)