Why is Idols of the cave most dangerous?

The idola specus are prejudices, by which individuals inappropriately extend norms or tenets that derive his or her own culture and social group, or to his or her own preferences. Racism, sexism and, more generally just "biases" are examples of idola specus, but the concept goes beyond them to the criticism of all forms of irreflexive subjectivity or individual predisposition.
en.wikipedia.org/wiki/Idola_specus

This sort of bias is most dangerous because it is invisible to the person involved. He thinks he is merely obeying laws of the universe.