My issue with Bernie Sanders is that he's a left wing populist that rarely passes bills and uses his influence to push other Democrats into taking very unpopular policy positions. Elisabeth Warren, who less successfully embodies the populist left, is very similar. Warren, however, is one of the least valuable Senators in the Democratic coalition, running considerably behind Harris because she's not very popular, liked, or have good policy ideas.
Sanders is a lot more interested in being a cunning politician than he is working with Democrats to win (which I very much want). It's why his 2016 supporters included Tulsi Gabbard, his former press secretary Briahna Joy Gray is a podcast grifter who attacks Democrats because they don't embody her extremely unpopular (and unserious) positions, and several other cranks and grifters in his orbit that do more to harm Democrats than they do to help. His entire orbit is full of people who, similar to Trump, aren't serious about policy but are serious about turning their influence into a self-serving platform.
I could go on and on about Bernie Sanders, but I think my point has been sufficiently made. His stances are not helpful, his policy is not serious, and the people around him have been toxic.
My best guess is that they lead the industry in rejecting claims because their profit margins are razor thin. The solution to healthcare is not Medicare for All, which I assume you are implying.
Yes, it would help. But the reality is, American healthcare is expensive because doctors in the United States get paid a lot more. Healthcare in the UK, for example, is cheaper because nurses in the UK make 37-42k a year and doctors make 100k a year. Nurses there make half as much in the UK as they do in the US, while doctors make 70% less. The same would be true if you looked at all other countries - their providers are simply paid much, much less. In addition, the United States is old, so the costs will only go up over time.
That's why healthcare is expensive here, not because of industry greed. But because Bernie is a dishonest grifter and cunning politician, he conveniently leaves out the part where huge swaths of the healthcare industry are going to have to see their pay cut dramatically in order to realize the cost savings other Western countries enjoy.