Question

why is a jack screw provided with a long arm

Answer 1

I hope it is help you. .. bro
The jack screw provided with a long arm because in order to produce maximum turning effect of the screw jack the force is applied at point of body for which the distance from the axis should maximum so that the given force provides maximum torque to turn the screw jack.
since torques = force * perpendicular distance .......
here torque is directly proportional to the distance that's why for maximum turning effect jack screw provided with a long arm....

Answer 2

A jack screw is provided with a long arm in order to have the effort less than the load. Therefore, the long arm of the jack screw, the more force it can apply.