Confirmation bias has been with us forever, but the "media" have figured out the economic value in providing it. How many folks make at least some effort to check "both sides" of some news item? Or even are aware that a thing happened because their preferred outlet didn't cover it?
Add in social media and you have a magnifying glass on opinions. We used to barely know, if at all, what our neighbor thought about politics, much less that of someone living hundreds of miles away.
Now we have folks who believe the NYT is entirely an outlet for DNC Talking Points but who have never read it, or haven't any time recently. I have encountered a few who believe Fox News has gone far left, and others who believe MSNBC is pretty right wing. This says more about them than the outlets I think.