Because if these primal beliefs do not have  any evidence or explanatory power (beyond explaining why we feel a certain way) then they are of limited use (and probably only exist because it helped our ancestors survive to believe in them), but they can be a huge distraction in our quest for the most likely and truthful explanations.  Arguments that start: “It feels right to me that…” or “I choose to believe that…”  while applicable to our daily life, should be rejected when it comes to the big questions of life.