Consider an example. A Raspberry-Pi can only give 23ma max current, and the motor will pull 600 ma current. So your Raspberry will go up in smoke. Moreover, during breaking, the back emf from the motor’s inductive coil will fry the R-Pi.
So you need a transistor to provide the extra current. And to block the back emf, you need a freewheeling diode.
And since it is a bipolar stepper motor, i. e., the current direction needs to be reversed to rotate in the opposite direction. For that, you need an H-Bridge, i. e., 4 transistors. And 4 freewheeling diodes.
The power dissipation will be enormous if you use just any transistor (BJT), so you need low power transistors.
Manufacturing 4 transistors and Diodes are more expensive than developing an integrated driver chip, with additional protection, such as voltage regulation, overload, etc.
That is why you can not use a stepper motor or any other dc motor without a driver directly with a microcontroller.