The illusion that they're impartial and unbiased is important to keep up the ruse. If there was only a single state owned broadcast, most people would realize it's bullshit. With the current setup, people watch CNN, MSNBC, and maybe a bit of Fox, then think they have a complete, unbiased, and multifaceted view of politics from all sides. On top of that, the mainstream media is calling for social media censorship. That helps kill off the alternatives to mainstream news sources, forcing people to use them.