Why did the classical economists believe that any long-term unemployment had to be voluntary?

Classical economists believed that long-term unemployment was voluntary because they assumed that labor markets functioned efficiently and that wages would adjust to clear the market. They argued that individuals would choose not to work at prevailing wage rates if they found those wages insufficient compared to their reservation wage, which is the minimum income they would accept. This perspective held that any unemployment beyond short-term frictional unemployment resulted from individuals opting to remain unemployed rather than accepting available job offers.