Because they form their worldview around their own interests, without realizing it. The label of progressive does a lot to assume moral right and inclusiveness which hides biases of self interest. Many of the beleifs people hold were not formed from the effort or reasoning by them but adopted form the authority of progressive ideology. Religion works in much the same way. It takes concerted effort and emotional regulation to build a rational and consistent worldview which is something most people, even progressives, rarely attempt.