Why don't Leaders need a Positive Televisual Image?

-Sheffield Rally happened a Week Before the 1992 Election, so its hard to say that this was why there was Different Levels of Support between the Polls and the Results of the Election -Lib Dems increased their Vote Share by 1% in 2010 and lost Seats compared to 2005 [Increased vote share however] so the Debate did little -Miliband did quite well when Interviewed by Jeremy Paxman in 2015 Election. Still lost. -Johnson vs Corbyn Debates in 2019 was seen as Dull and Repetitive with no Obvious Winner