That's because you aren't counting all of the super duper extra-secret Marxists!
Which, by the way, I'd never heard about until Obama was elected. Weird coincidence. I am sure there is nothing correlated to a certain unique attribute of that President compared to everyone that came before him. No need to look into it further.
I would assume that Biden said that because he knows it is not practical and would be a waste of his limited time and political power.
Obama made a similar compromise in not pushing for single payer but instead focusing on the compromise that became the ACA. He used basically all of his first term to get that passed, and while it may not have gone far enough for a lot of progressives--including myself--I sure am glad we have that instead of nothing!