The fuck are you talking about? First of all, primaries exist. Secondly, with only a few notable exceptions, Democrats favor single-payer. The original ACA was a government healthcare bill with a private option but Joe Lieberman was the holdout and gutted it. Stating otherwise is intellectually dishonest and deliberately ignorant, or at worst a malicious rewrite of history.
It's not stupidity, because over 99% of us didn't have a choice. It's greed. Greedy little piggy leaders of major systems designed to fuck over everyone but them.
My default response to this is what was voter turnout for the last election? People are fond of referring to the 4 boxes: soap, ballot, jury, and cartridge, but they don't bother with the second.
I've literally never heard a single person refer to these 4 boxes, so I must take issue with your assertion that people are fond of doing it. You're also seemingly discounting innumerable factors, like the hundreds of billions of dollars spent on distracting and deceiving people into both voting against their best interests, and not voting at all.
In short, your default response is a reductive and unhelpful platitude. Not trying to be a jerk (it just comes naturally). Best of luck.