With flexible wages, this wouldn't happen - unemployed workers would bid wages down. The Natural Rate of Unemployment Unemployment will always exist because people have to spend time searching for suitable jobs. Regulation of labor markets tends to create additional unemployment. At any given time, some people are finding jobs, others are leaving them. Demographics play a key role.
Younger people are less certain about what they want to do, and are changing more rapidly.
Women are much more likely to quit or start work for family-related reasons. According, more young people and more women typically lead to a higher natural rate of unemployment. Similarly, more highly educated workers change jobs much less. Being more specialized, they have probably already found a good match. Regulation can greatly increase the natural rate.