It seems that lately, just about everybody refers to a stepper motor with an attached lead screw as a linear actuator.  Techically it does what a linear actuator does, moves a component in a straight line, but that's not how a real linear actuator works.

In a true linear actuator, the motor and the movable platform are integrated.  The rod, or more likely the rail it traves on, is stationary, and the traveling block has the stepper motor and rollers inside it.

This first video shows how a lead screw actuator works, and yes they call it a linear actuator.

https://youtu.be/uewMphsBamk

But this video shows a true linear actuator.

https://youtu.be/-dMKXUPLMW0

While they both do the same thing, the linear actuator is more complex and costs more.  Hope this helps

Indeed, the same nema types stepper motors https://www.omc-stepperonline.com/nema- … 0674s.html, with a linear actuator https://www.omc-stepperonline.com/nema- … -250e.html will be more expensive. Is that the reason for linear motors' rare use on 3d printer?

Not sure where you are getting your information from but such motors are often used in the Z axis of most printers.