Back Emf of dc motor
Because electric motors are pretty efficient.

When a motor spins, it acts like a generator.  When it spins due to a applied voltage, the spinning generates a voltage internally that is opposite of what you applied.  What keeps the motor spinning is the difference of the applied voltage minus this internal EMF.

This is why the no-load motor speed self-regulates as a function of voltage.  At first the motor is not spinning, so all the applied voltage goes to making it spin.  As the motor speeds up, the back EMF becomes stronger, so there is less and less voltage left to spin the motor.  Eventually, the motor goes so fast that the back EMF cancels enough of the applied voltage to only keep it spinning at that speed.

The more efficient the motor, the closer that back EMF is to the applied voltage.  Since the motor is theoretically doing no work, it should take no voltage to keep going.  Real motors have real friction and other losses, so it will always take some finite fraction of the applied voltage to keep it spinning even with no load.