Because, to first approximation, this is true. Every organization, every person has their own biases and agenda. I'm not sure why Americans believe that objectivity in news reporting is even possible. Other countries don't seem to have as much of an issue with this, since you typically have news sources that are either owned directly by the government or are published by political parties.
This is not binary. There's news podcast I listen to ("Raport about state of the world" - Polish only sadly), and host always tries to advocate for both sides when asking questions and often there are guests from the both sides, that present their point in calm, collected manner. Then there's our state TV, which will tell you that EU is devil, opposition is devil, basically everyone is devil apart from ruling party,…
Sometimes, something just isn't true and the other side doesn't get equal attention to defend their points. You can calmy explain how lizard people inside hollow earth run Hollywood to turn our children into gay frogs, but these people shouldn't get any air time, not even to be made fun of.