If planning involves working out details for the future

If planning involves working out details for the future, why does it not ensure success?
or
’Planning is not a guarantee of success’. Comment.

The success of an enterprise is possible only when the plans are properly made and then implemented. In order to achieve goals, plans need to be translated into action or they become meaningless.
Managers usually rely on previously tried and tested successful plans and like to use them in every similar condition/situation because of the view that the plan has worked before too. But, business environment is dynamic and rigid plan is not a guarantee of success.