Like what do these posters think people did for decades? Where did that all lead us? Right to fucking Trump.
I think liberals like these people do genuinely believe in some kind of divine intervention/perversion of reality/magical curses, rather than a series of cause-and-effect chains that lead towards bad outcomes.
On the right wing, you have the "our memes and pepes got Trump in office! we did that!" which is bizarre and self-important and delusional but does at least acknowledge that there was a cause and effect. whereas liberals look at 2016 and go "Well, look. In 1981, Reagan cast a magical curse on America that made things worse and worse every year. And then we got to Obama and the curse broke, but then Trump cast the curse again in 2016, rolled a nat20 on a persuasion check for convincing the Republicans to be freaking fascists, and now we're screwed again. Maybe if we elect Harris, the curse will be lifted again." There is no inkling that somehow Obama could have led to Trump. Or that trying to elect Hillary Clinton, with one of the most uninspiring political platforms in human history, could have led to Trump. That electing feckless and useless Democrats for the sake of "lesser-evilism" could, perhaps, actually be the cause of Republicans gaining power later and doing awful shit. If Trump is polling better than Biden, then perhaps instead of blaming X or Y group and insisting that they just aren't paying attention to all the great things that Biden is done, you need to sit down and accept reality and go "Huh. Fuck. Maybe Biden does fucking suck, actually. Maybe electing Joe "nothing will fundamentally change" Biden in 2020 might in fact have created the material conditions for Trump's return to power. Hm. Shit."
Whereas a person with political curiosity and a willingness to learn in 2017 might be like "Hm, this is bad. Trump is president and he sucks. I think we need to take a hard look at how we got here. Let's do some reading and... oh, huh! It seems like Lesser Evilism has been a long-running current throughout modern American electoral politics and has lead to the shitty outcome we just saw, because voting for a bad candidate instead of aiming for societal change against the system itself has never actually fixed anything and is a mere can-kicking exercise! We need an alternative theory of how to do politics that will benefit minorities." One might even become increasingly internationalist and understand how the US fits into global geopolitics.
But no; as western "leftists" like on tumblr are alienated little treatfiends, they can only conceive of individualistic solutions. In their own words: you either vote, or you firebomb supermarkets. They cannot even imagine the creation of alternative political structures with other people, they are so deep in the mire. As you say, they can't even imagine going out door-knocking for Biden and now Harris as part of an organization.