For the same reason a regular Progressive who believes in Medicare for All isn’t going to opt out of private health insurance and give 30% of their money to the government. There is no program in place for the money to go to. Under a progressive tax system, they might not owe all that much, if anything. And the goal is to put the program in place and require everyone to participate. Look at the success of the Toronto Protocol banning CFCs to save the Ozone layer. How do George Carlin’s arguments apply there? “If you are really worried about the hole in the ozone layer, just stop buying hair spray, and stop pestering the rest of us!”