The premise isn't that the folks with WikiPedia can rebuild modern society. It's that they literally can't (even if they had better knowledge resources), but would still have a survival advantage from having a little bit of the old knowledge. The fact is that if we lose our modern society, we'll never be able to build it up again. We've dug up all of the easily accessible resources, already. Scavenging from the good old days is the best any post-apocalyptic society can hope for, as bleak as that sounds.