Because it's too late. What do you think you can achieve in seven months? You'd have to unite leftist Boomers, Gen X, Millennials, and Gen Z and convince them that your plan (third party voting or "industrial action") is better than the relatively safe option of simply going out and voting for Biden.

And not only that, but thanks to FPTP, convincing people to join you also means convincing people that it won't throw the election to Trump, whose party reliably turns out to vote for more Republicans.

Unless you know something I don't, we're about four years too late to build the momentum needed to make dramatic changes.