Howard Schultz simply believes that he can provide solutions that can make our country better than it is in 2020. He also wants to do it as a Third Party Candidate and NOT as a Democrat. That is where the problem for the Left comes into play.
He wants to make changes without becoming as radical as they are. People have seen that the Democrat Party has become a progressive cesspool.
The Left is scared that this is Ross Perot or Ralph Nader all over again. They do not want to see Donald Trump succeed like he did in 2016. They cannot live in a world where he is the President for four more years.
Howard Schultz is not a bad guy consuming himself with the bad ideas of the Democrat Party as a whole. However he does represent a true threat to the Democrats regaining power which is the only thing they truly care about.