And it's rampant on both sides. It's gotten so I don't believe anything I see or hear anymore - unless the conservative and liberal media agree on it, then maybe it's got a bit of fact in it.
Read Mark Twain's take on the media (newspapers of the day) and it's fun how much is just as applicable to today, though it ebbs and flows. The 24 hour cable news cycle put us back to the outrage for dollars/over sensationalizing everything and then social media poured gasoline on it.
Both sides do it for the same reason competing newspapers in the pre-Civil War era were overtly partisan. Captive market. Some do it to push change, but most do it to push change into their pockets. In most of the broadcast television age, news was viewed as a loss leader and a prestige product. Now it's a major profit center, and you need outrage, sensationalism, sex, or fear to keep eyeballs. I bet most people here are old enough to remember when the weather forecasts all started being marketed as "Storm Teams" as an example.