They are.
But not with some insipid elite centrist with nothing to say, like Buttigieg or worse still, Beto.
Everything about them, before they have even opened their mouths, signals contempt.
Ordinary people do not think Sanders' proposals are extreme (on the rare occasions when they get to hear what they actually are).
They assume Sanders is extreme because there are millions of dollars invested in telling them this, every single day.
Elites, from both parties, do think he is extreme, because if he won, it would cost them money, but more importantly, because he (and especially, the concept of mass political engagement that he represents) challenges their political, intellectual, and moral authority.
If Sanders ran against Trump and lost (which, to be sure, is very unlikely), it would be because Democrats, and not Republicans, failed to turn out for him.
If he did win, the Democrats would be infinitely more conniving and vicious in seeking to destroy him than the Republicans, in exactly the manner (albeit for different reasons) that they were with Jimmy Carter.
Setting aside how his own party would react, he is nonetheless probably the best chance we will have in decades, and maybe ever, of winning back disaffected suffering people in places
like West Virginia, which for decades voted Democrat every single time, but which abandoned the party when leaders like Joe Biden and Bill Clinton abandoned them. We are well on the way to losing Ohio, with far more votes, forever, for the same reasons.
When we say that people who voted for Trump are not 'gettable', what we are in effect saying is that we have given up on the political future of the country altogether, because nothing, from Klobuchar's rote babbling about 'means tested marginal tax credits' or whatever, to Warren's vastly more ambitious (and appropriate) regulatory reforms, is even worth discussing so long as Mitch McConnell controls the Senate.
Winning the real transformational power required to meaningfully reform the country and ensure something like Donald Trump or worse, Mitch McConnell, never happens again, does not sound like a dinner party of comfortable well-educated coastal urbanites (and I am not excluding myself here) preening about how much cleverer they are, and how the other side is too dumb to know up from down and thus deserves every misery it gets.
It sounds like this: