People's beliefs and ideals tend to align with their self-interests. For example it was quite well accepted among scientific circles in colonial America, that

People's beliefs and ideals tend to align with their self-interests. For example it was quite well accepted among scientific circles in colonial America, that black people are not really humans or conscious. Therefore it's OK to exploit them and keep them as slaves. It is also currently quite well accepted that animals we eat are not that conscious. Although oddly, if we keep them as pets, they're super conscious sometimes. The rules of cognitive dissonance can grow arbitrarily complex, to permit us to do what we wanted to do anyway, but also sleep well at night.