Why do we still have political conventions? There is a legal requirement that they actually sit down and have the formal vote on who their nominee will be, but that does not take days of speechifying and pageantry. If another political party like the Greens or Libertarians tried to get their conventions on prime-time teevee night after night they’d be laughed at. So why do the two parties get so much unfiltered airtime?
Because people watch it.
About 40M Americans watched Obama’s acceptance speech in 2008, and nearly 2/3 of all Americans watch at least some of the conventions. That’s about the same as the Olympics, generally speaking. People actually want to hear the candidates speak without filters, and they want the party to tell the world what it stands for.