An external force is required to change an object's linear momentum. The same concept applies to rotational behavior as well. An external torque is required to change an object's angular momentum. No torque is needed to keep the Earth rotating about its axis. A torque would change the rotational motion.
That said, such external torques exist. The moon and sun act through the tides to slow the Earth's rotation rate. A day was considerably shorter when the Earth first formed than it is now.
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