Because you couldn't. In the ancestral environment, there weren't any scientific journals where you could look up the original research. The only sources of knowledge were what you personally saw and what somebody told you. In the latter case, the informant could be bullshitting, but saying so might make enemies, so the optimal strategy would be to profess belief in what people told you unless they were already declared enemies, but base your actions primarily on your own experience; which is roughly what people actually do.