My answer: This idea is metaphysics, not science. In other words, it is an assumption, not a finding. The assumption of this idea is that the assumptions of science are ultimate.
So, the assumptions of biology, of brain research, exclude all supernatural factors, exclude anything which is not detectable or measurable - and therefore, not matter how much brain research is done, none of these things will ever be found because they are not included in the methods.
Brain research has not failed to discover the soul, it has assumed the soul is not relevant.
In history, when brain research believed that the soul was potentially detectable, people would often say that they had located it - for example in the pineal gland.
In sum, if you believe (before doing any research) that chemical reactions and wiring are a complete explanation of all ideas, then that is what you will find.