Why are stepper motors used in 3D printers?
FDM 3D printers need to do repeatable moves. Servos might give better and more repeatable torque, but they don't give very precise movement without the need of an encoder system. This makes Steppers cheaper and easier adaptable in most cases, even though there are some printers that use different motor systems.

Note that different printers like SLS printer do use several simple motors, especially for the system putting down powder, and the tilting system in the new mSLA Prusa uses - afaik - a simple motor doing one rotation per tilting motion.