I think employees feel that the manager is only asking for the feedback, so that they can just complete the process. There is no motivation for the employees to give real feedback, as its not really worked upon.
Employees might feel that their feedback and opinions would not be taken in a positive manner and hence they might hesitate from giving honest answers.
A lot of times, employees have a fear of their feedback not being anonymous. This sometimes leads to either pretentious feedback or skipping the feedback surveys altogether.