I still don’t believe in the media bias of the mainstream media because you’d never have things like Global Warming being a question. You’d see more about the insane incarceration rate and the obvious bias that police practices perpetuate (like the fact that whites are just as likely to use drugs as blacks, but are arrested and prosecuted at a much lower rate). I think if you looked at media coverage in more liberal countries, you’d find their reporting to be significantly more liberal.
I will say, however, that you make a compelling point. Newsrooms are no doubt more saturated by those with liberal leanings and it probably does color some of the ways they attack stories. Still, they know where their bread is buttered and what they’re allowed to cover.