The problem is that UI/UX people are product and design people whose backgrounds were predominantly in art or similar. 99% of these folks have no background psychology, human factors, information science or the cognitive sciences.
Like any other kind of employee, you the stakeholder are responsible to get them trained in the role you delegate to them, or to evaluate that they meet your needs before hire. Designers I know do art for fun, but at work they design to solve business problems. They have a lot to say about this topic. In any case it isn't due to some character flaw on their part.
What can be expected when you hire these people?