Stewart is smart enough to know that he doesn't know what he doesn't know and humble enough to admit it. He's a very popular entertainer and pundit. He has some great takes on the state of American political discourse. But none of that qualifies him to be the President of the United States and he knows it. That's not to say he couldn't win. After all, Trump knows nothing about how to run a country and he got to live in the White House for 4 years, but we all saw how that went. People want Stewart to run just for the debates. But he knows better than anyone that he isn't even close to being qualified for the job.