While it is sometimes difficult for even the most ethical journalists to filter their own prejudices, this is not the cause of the bias. Journalists today insert their opinions intentionally to try to sway their readership to their views. Journalism has evolved into advocacy.
A recent Prager video titled "Why no one trusts Main Stream Media" presented by Sheryl Atkinsson covers the problem admirably. Well worth the few minutes.
The consequences for our society are dire. A free society require a vibrant free press. The media's role is to gather and report facts without fear or favour, without bias and free of personal opinion. While there is room for personal opinion and debate, it must be clearly delineated from the factual foundations. This allows the audience to critically review the information presented and form their own views. It generates a 'marketplace of ideas' allowing the survival of the fittest.
Alas it seems our media has caught the twitter/facebook disease whereby like minded audiences listen to like-minded journalists entrenching the same ideas in an endless loop. Advocates of an idea become so entrenched they cannot even consider an alternative view and will even resort to violence to prevent it being presented.
In 1839 Edward Bulwer-Lytton noted "The Pen is mightier than the sword" recognizing that the written word had more power than direct violence. How ironic that in today's world those who wield the pen have created a society that is willing to use the "sword" to control the "pen".