I do not understand why people take so seriously what entertainers say about politics, world affairs, health issues, and education issues. Is this true in countries other than the US? Has it been true throughout history?
Apparently yes.
Shakespeare's plays were used to attempt incite rebellion during his lifetime and Aristotle warned against undue popularity being given to certain types of music and musicians. However the cult of worshiping fame has become much more powerful in the 20th and 21st century with "famous" people's endorsement being sought by politicians and the even worse either politicians trying to be come famous as entertainers (eg Boris Johnson) or Entertainers switching to become politicians such as Ronald Reagan, Arnold Schwarzenegger, Volodymyr Zelenskiy and Donald Trump.
To quote the old TV series "Not the 9 o'clock News" "Getting your physog on the googlebox" can open all sorts of doors.
President Barbara Adams You're telling me that the entire population of Great Britain went and elected a deranged psychotic to the highest office of the land? [pause] Again?
Whoops Apocalypse (1986)