While people could be rational most of the time, they often don’t choose to be. This is especially true when it comes to religious faith. Sure, few religious people utilize their faith in dealing with common problems: when they have a flat tire, they don’t get out of their car and kneel down and pray for a miracle fix; when they have physical maladies, with notable exceptions (such as Christian Scientists), they usually consult physicians and don’t leave a cure solely up to their god. All in all, most people these days generally confine their reliance on faith to their minds and don’t let it rule their plans of action.
Why then is faith still so popular, even in advanced societies? It’s not because faith is actually useful or effective. In large measure, it is because faith is promoted by its champions by associating it with desirable things such as art, marriage, love, and even science (as in the case of the Templeton Foundation and its cadre of religion-friendly scientists). The concept of free will, too, is left to the domain of faith, because atheists tend to be materialists, a stance that for many precludes contracausal free will or causal agency in the natural world.