Most of them haven't needed them due to reasonably fair coverage from the media. In the face of unremitting negative coverage then supporters of President Trump resort to paying for positive ads themselves
https://www.washingtontimes.com/news/2017/dec/12/media-bias-continues-90-of-trump-coverage-in-last-/
"The working relationship between President
Trump and the news media shows no signs of improving. Journalists continue to appear aggressively hostile toward the president, according to
research released Tuesday by the Media Research Center, a conservative press watchdog.
The new study of major broadcast coverage reveals that 90 percent of statements made by reporters and nonpartisan sources the last three months on ABC, NBC and CBS evening newscasts about
Mr. Trump and his administration were negative."
https://www.npr.org/2017/10/02/5550...trump-more-negative-than-for-other-presidents
"Compared to other recent presidents, news reports about President Trump have been more focused on his personality than his policy, and are more likely to carry negative assessments of his actions,
according to a new study from the Pew Research Center's Journalism Project."