Today's world is vastly more complex than it was in our ancestral environment, and that makes assessing truth very hard. It's not possible for anyone to directly access all primary sources, so we have no choice but to set up some kind of division of labor in the assessment of truth. Scientific peer review is one example of such a division of labor. We trust peer reviewed publications not because peer review is flawless, but because we have to do something to triage the myriad firehoses of information that exist in today's world, and peer review is the best we've been able to come up with so far.
Handing over the arbitration of truth to unaccountable private corporations is, like peer review, far from optimal. But we have to do something or those who are out to deliberately deceive to advance their political agendas will win. It is always easier to destroy than to create, and it is easier to promulgate a well-crafted lie than the truth. Those are the facts on the ground that we as a society have to grapple with. It's a hard problem and none of the solutions that have been proposed are optimal. But that's no excuse not to try to do something. If we don't, society will descend into chaos and authoritarianism. Fact checking by social media platforms is far from optimal, but it's a hell of a lot more likely to advance democracy than survival of the fittest con man.