Oh, wait - you really think a majority of Americans will vote for a Socialist? You need to get out more. You see, the far rightwing nutjobs in this country are a minority - as is the far left. The majority of people in the USA are middle-class and middle-of-the-road. They don't want to rock the boat that much. They vote for the candidate who seems the least extreme.
I had friends who voted for Bush - twice. When I asked them if they were voting for John McCain, they said, "Hell, no!" and the reason why was Sarah Palin. They voted for Obama not because they were Democrats or Liberals. They voted for Obama because he seemed a lot more rational and middle-of-the-road than the crazy show at the McCain/Palin camp.
If the choice was Saunders or Jeb Bush, I can guarantee you my friends (and the majority of Americans) would vote for Bush. He seems more middle of the road in comparison. But if the choice was Hillary or Bush, she would likely win. You can decry this as "unfair" and all, but it is the honest truth, and the polls back this up. And no, the polls are not wrong in this regard. Sanders has no hope of being elected President. The most he could hope for is to run as VP and then hope....... well you know.
Third party candidates are just spoilers!
EDIT: It might not be Jeb Bush. It could be Rubio. Is there really a difference? Of course not.