They can't because some people have a block in their thinking process that prevents them from rationally contemplating any thought that goes against their own prejudices.

They'll scream CALORIES IN CALORIES OUT until they die from apoplectic rage rather than accept any concept that considers that there are more fine-grained factors at play in weight loss.

Saying CICO is the only thing that matters is like saying "Apples fall from trees because of gravity" is the grand unified theory of the universe.