Because (a) most people simply don't have the time or skills to fact check every single piece of information and (b) even if they did it assumes that they have the awareness to do so. Both assumptions have been proven wrong again and again.
What we can do instead as a society is force the the billion-dollar social media companies to take responsibility for the content that they chose to display and the ads they chose to serve. Same as newspapers do, television stations do, and every other media format do every single day.