Because ego has wrong perceptions or wrong views which cause unnecessary afflictions and incorrect thinking. And true self has the right view which means the absence of all views or all perceptions. BTW, simply the wrong view means "I am special and separate". If we are mindful and concentrated, we can attain insight (the right view). And when the right view is solid in us, we can have the right thinking.
Thay said, "Right Thinking reflects the way things are. Wrong thinking causes us to see in an 'upside-down way'" and "Non-thinking is there. We are in deeper contact with reality, free of words and concepts." So, Right Thinking means Non-thinking. I feel that these words are applicable not only to an enlightened person but also to all animals, plants and minerals. Because they don't think and are always mindful. All the elements of humans, animals, plants and minerals are the same, as water, air, earth and sunshine. And each particle and energy of the elements is enfolding consciousness. So, the disintegrated human body and consciousness must be there even inside of the rocks.
El Capitan, Yosemite Valley Photo by Leisure Escapes