Why do we believe what we do? ===
We believe what we do, because of our natural environment internal and external - genetic and circumstantial - instinct and culture.
Why do they believe in the supernatural? ===
They believe in the supernatural because they have either been taught that it is the truth, or have had some later experience that caused them to believe it was the trust or, they need to believe, because they fear otherwise.
What are the secular parallels? ===
Secular people believe what they do, due to their natural environment... education, experience, fear of religion!
We don't want apologetic arguments pro supernatural belief. ===
I don't know about apologetic arguments - the facts are the facts and we can only state them.
We want to replace the need for such an argument. ===
I'm not arguing, I'm stating :)