The world in which we operate is extremely dynamic and constantly changing.
What can go wrong, goes wrong, and at the worst possible time of course...
The actual time taken to perform a task is likely to be different to the estimated time.
The actual capacity available may well be different to the planned capacity.
These are just some of the many factors that cause things not to happen as per the plan.
The changed conditions are not reflected in the plan and it cannot be executed in its current form.
People work on what they think is best and you are forced to re-plan using the current conditions.
By the time you have finished re-planning, conditions have changed again so you are stuck in this vicious cycle.