Because it requires time for the force applied to reach all parts of the machinery.
When two solid bodies, moving slowly, strike against each other, what takes place?
They are pushed off from each other in opposite directions.
Which body is pushed off the more rapidly?
The one having the less weight, is pushed off the more rapidly.
When the oars of a boat push against the water, what takes place?
The boat and the water move in opposite directions.
When the wings of a bird strike the air, what takes place?
The bird and the air move in opposite directions.
How can a duck or a goose swim in the water?