On the simplest level, because the very idea of faith requires going beyond the point where you are able to operate on pure reason. For topics that can be understood using experimentation, faith is not required.
"I believe in prayer" and "I am going to determine my religiosity based on the outcome of this experiment" are incompatible beliefs. Since we know the later is true, we can assume the former isn't.
"I believe in prayer" and "I am going to determine my religiosity based on the outcome of this experiment" are incompatible beliefs. Since we know the later is true, we can assume the former isn't.