Because this Idea of a measuring stick is really flawed.

A measuring stick that grows with your child will never tell you that your child grew in height.

Body weight scales that somehow adapt to you gaining or losing weight will never tell you anything about your body weight.

A thermometer that adapts to the temperature will never tell you if it's warm or cold outside.

Thus a money that expands with economic growth will never give you the benefit of realizing not to mention benefiting from productivity growth.