People are unskilled at maintaining an adequate degree of agnosticism and uncertainty. Agnosticism and uncertainty about an issue demands cognitive engagement and investment. It is easier and cognitively cheaper to simply default to heuristics and stereotypes.
The tendency towards great confidence and certainty in the truthfulness of otherwise contingent or untested heuristics and stereotypes is an adaptive attitude to preclude cognitive investment in that which we know to be uncertain.
If we did not assume truth, we would have to make the very cognitive investment we are seeking to avoid.