The key reason why there is always some unemployment is because the labour market is constantly churning.

The key reason why there is always some
unemployment is because the labour market is
constantly churning.
New jobs are created and old jobs die; and some people
move into the labour force and some move out of it. This
churning creates unemployment.
We distinguish among three types of unemployment:
• Frictional unemployment
• Structural unemployment
• Cyclical unemployment