People absorb politics from our social environment. We judge what's good and bad, what's controversial and uncontroversial, based on our models of the political discussions we've heard among peers and what we imagine they'd say. Every Twitter user I know comes to believe that the political dynamics on Twitter are a reasonable approximation of the political dynamics in the US, no matter how much they repeat the mantra that Twitter isn't real life, and this leads them to repeatedly overestimate how much people support crazy niche positions or care about esoteric issues.
I agree, it's amazing how many Twitter users get their sense of politics warped by the platform, and I think this is a big part of the reason why it's appropriate and important for those of us who realize this to loudly and vocally shun the platform. I've called out friends and acquaintances in person who still use it, and I'll continue to call it out online (for example, any HN post that is a link to Twitter).