The unemployment rate has a downside limit, if for no other reason than unemployment, in part, reflects voluntary periods of job search and other frictional unemployment, and includes people whose skills are not well adapted to work today and would be very costly to employ.
BIS Review 93/1997  In addition, there is a limit on how many of the millions who wanted a job but were not actively seeking one could be readily absorbed into jobs -- in particular, the large number whose availability is limited by their enrollment in school, and those who may lack the necessary skills or may face other barriers to taking jobs.