Why? With how confidently you state that, I'm rather curious what reasons you have.
If you want a longer answer, George Orwell penned an eloquent one all the way back in 1944: https://www.orwellfoundation.com/the-orwell-foundation/orwel...
As such, you end up with a large cohort of people believing immigrants eat their pets, vaccines have microchips in them and are more harmful than the diseases they protect against, 5g towers cause cancer, chemtrails are a thing, and trickle-down economics benefits working people.
Now, I may ultimately accept the idea that no matter what we do, we're always inevitably screwed, and even the smallest attempt to curtail speech will always end in an even worse outcome (like how there exist some infinities larger than others), but even I get a little uncomfortable being that nihilistic.
It is not that one has to accept that we are inevitably screwed. That assumes that no amount of work or effort can address social problems peacefully, and that the only way for a functional society is through force.
Im hoping that current decades of polarization and championing of censorship will end up resulting in similar conclusions as the UN did after world war 2. Censorship and violence only breed higher quantity and intensity of censorship and violence.
We've had decades of Fox News and the like declaring things like "War on Christmas" and "War on Christianity" to make people in the majority feel like victims, presenting immigrants as subhumans that take jobs and commit crimes as they invade our country, and presenting trans people as deviant threats to our children, trying to make them all trans, too.
This is a completely fictional world, but such a large number of people have become believers that they've now been able to take over political power.
As a result, we get state laws that directly attack freedom of speech via book bans and scrubbing school curriculum of anything parents deem objectionable, which can include innocuous things like acknowledging gay people exist or that the civil war was fought over slavery. We also got our current administration, which has used lawsuits and other threats to attack any speech the president doesn't like.
I don't see where it gets better anytime soon - and I think it's a foregone conclusion that it's going to get a lot worse before they do get better, because a large cohort of people are cheering it on.
And before anyone chimes in that this is a both sides issue: I've yet to see actual legal action taken by the left wing to curtail speech. Instead, I've seen social pressure levied - largely in the form of freedom not to associate with individuals or businesses that engage in speech people find objectionable. This is the correct way to engage in an environment of free speech, even if I find it distasteful how far it's been taken and how petty it's been in some cases.
I'm not really advocating for censorship myself. Ultimately, I'm merely reflecting upon how an environment of nearly absolute adherence to free speech has been eroded by a number of bad actors utilizing propaganda and lies to chip away at that very free speech over the decades, bringing us to a point where we're sliding down the very same slope towards destruction of freedoms that free speech absolutism was intended to prevent. The whole exercise feels like a Catch-22, hence my prodding for something a little more concrete yet specific than "censorship = bad".
It is fairly common belief that lies and propaganda is stronger than truth. However rather than see it as a fundamental part of nature, I would propose an alternative theory. Lies and propaganda is a symptom when social trust in society start to break down. You could ban Fox news or any other right-leaning media and there is little to no evidence that society would be any different.
The idea of free speech absolutism is an concept that people build to attack free speech. Free speech is about valuing and believing that people should be free to hold and express their opinions and beliefs without fear of retaliation. Fear can be created by law, by mobs or by those who hold some form of power like employers, but regardless of method the result is the same. Society need to value the idea of free speech. Absolutism has nothing to do with that. The idea that people should not fear the government for beliefs they hold, but should fear their employer, is inconsistent with free speech as a human right.
If you are looking for something more concrete, I would point towards research that that looks at social trust and its roles in conflicts. There exist a fair amount of research on this topic, some which is left politics and others which is right politics. One major finding are the importance of shared values. What kind of values those are is less important than that they are shared. If they aren't shared, then the next most important part is that they aren't shoved into the face of people who do not share them. Trying to stamp out opposing values, especially in a public and diffused way, has a long history of creating violence, fear and mistrust.
What they're sometimes guilty of (in my judgement) is one-sided reporting. E.g., regarding illegal immigration, providing sympathetic personal stories of illegal immigrants, but not of the persons hurt by illegal immigration.
It's also possible they've gotten better about this. I stopped listening years ago.