Why do planets rotate ?

Newton's first law of motion, the "law of inertia" tells us that:
"An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

So, to imagine an Earth that does not rotate is to imagine an Earth that either has not been acted upon by a force or has been acted upon by forces that cancel each other, exactly.  Neither case is very likely in the Earth's 4.54 billion year history.

Planets form by accreting matter as they pass through space.

Not all of that matter will hit the forming planet on a vector aligned neatly with the center of the planet.  Some will hit a glancing blow and put a rotation on the planet.

But it's also likely that the initial cloud of material will have a net angular momentum that causes a small rotation on the cloud.  Conservation of angular momentum tells us that as the cloud condensed, it would have to rotate faster - just as an ice skater spins faster as they contract their arms.

And then there are torques caused by other bodies, such as the sun, moon, Jupiter, and near-pass asteroids - tidal forces that affect the rotation of the Earth.