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.

In the ancestral environment you likely live more-or-less the same way your parents/elders did, so any advise they gave you was likely to have been verified for generations and hence good.

A believer in ev-psych would say something like "humans evolved language to manipulate each other, not to find truth".

Anyone want to come up with a theory about why not bothering to get things right was optimal in the ancestral environment?

Isn't sloppy science just a special case of the effect Eliezer described here?