To me this is sort of like saying why do we need seat belts when we could just have people go to the gym so they're strong off to push back an oncoming car. Well, you can't get that strong, and also you can't really educate people well enough to reliably deal with the full force of the information firehose. Even people who are good at doing it do so largely by relying on sources they've identified as trustworthy and thus offloading some of the work to those. I don't think there's anyone alive who could actually distinguish fact from fiction if they had to, say, view every Facebook/Twitter/Reddit/everything post separately in isolation (i.e., without relying on pre-screening of some sort).

And once you know you need pre-screening, the question becomes why not just provide it instead of making people hunt it down?