Their ego is in the way of good, clear reasoning. To a large extent, they are probably unaware of the role their background beliefs are playing in their reasoning. They are not willing to be objective. Their reasoning is generated from a psychological need to defend what they already believe. Piaget concluded that children think they are the center of the world.
Many adults think in roughly the same way.
The beliefs, values, attitudes, and actions of one's group are seen as better than those of other groups. According to Vincent Ruggiero, people who do not acknowledge a world-view become victims of it. The weak sense critical thinker will use reason in a limited way. They will use it only to defend their position. This is not the same as seeking the truth.