Why do humans so often exaggerate their perception of credibility?

Falling in love is often or even usually an example of that, and has the result of increasing fertility, giving the credulous a fitness advantage. A tribe with more credulous warriors may have a military advantage by being more cohesive and easier to lead.

The willing suspension of disbelief has been a successful genetic mutation. Genes learned to meme.

Because the emotion comes first, reason second. The primary purpose of reason is to rationalize emotion.

In fact, I suspect that the entire sensorium (sight, sound, etc) is an abstraction of emotion.