For a lower stakes example, why is the organic produce in the grocery store more expensive? If you took the labels off and mixed it all together with the rest, it wouldn't be. The value at sale time is derived from a meticulous record keeping process that makes faking provenance comparable in cost to just doing it right.
Provenance of human written knowledge comes from a lot of places. Just because something was written by a human doesn't make it accurate or non-garbage. But the labor cost in producing misinformation that's hard to distinguish from meaningful writing in the same domain, together with a lot of systems we have in place, makes evaluating provenance a tractable problem.