So people leave agriculture and go into industry. Employers will want to hire fewer people than want to be hired at the market wage. Mandated benefits, mandatory overtime, and other regulations play a bigger role. In other countries, especially in Europe, labor market regulations are typically very strict. You trade-off between the lost wages of searching, and the potentially higher wage you will earn if you find a good match.
Employers make the same trade-off.
Insofar as unemployed workers are engaged in useful search activities for unknown opportunities, it makes sense to view them as voluntarily unemployed. It is a much bigger puzzle if workers' best match is obvious, but unemployment persists. 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.