Truth is, they know Trump will likely win.
The level of polling and statistical forecasting the party insiders get is very different and far more advanced than the near-meaningless polls the public gets. It's easy to answer a phone call and say you'll vote for Biden, but actually getting off your duff to go cast a vote is a very different thing. The excitement level among Democrats for Biden is virtually nonexistent, but Trump has a solid base who will turn out for him.
The wild card in the election will be vote by mail. There is tremendous opportunity to cheat there, and frankly voting by mail is too easy. There has always been some intentional difficulty in voting, because the founding fathers wanted voters to be impassioned to vote and educated when they did. They wanted voters to take their responsibilities seriously, do their homework, and see it as a tremendous duty. What they didn't want is for overly emotional people to get up that morning and decide they'll go vote for one candidate because simply because they've been told to hate the other one.
One of the biggest issues we face right now is disinformation and the dumbed down voter. While this is not exclusive to either side; the Democrats, in particular, often have no idea of the real facts. Add to this that they are intentionally being driven by negative emotional issues. Abortion, a very skewed view of racism, and of course the incredible hatred that had been manufactured toward Trump. I dare say that more who will vote Democrat cannot give you a good reason why they will vote for Biden, only that he is not that awful orange man. This is the very type of mass ignorance that the founders were concerned about, and the Democrats have mastered the art of herding their sheeple. Of course, they have powerful allies in the media.