In normal conversation a closed loop system running any machine tool can be called a servo without regard to the exact motor type. So when we say servo motor, it can include any motor type and still be understood. Just language shorthand, but not entirely descriptive.
It really depends on the machine and machine manufacturer. Also it's about price point, steppers are cheaper. Some smaller mills do come with steppers, my little Emco Concept 55 has steppers, as do the Tormach machines, and the old Bridgeport Boss series came with steppers also. Where my Eagle (BP clone) and Haas have servos, as does my lathe. Most of the higher end and higher performance equipment comes with servos, both routers and mills. More accurate and more power. Steppers loose torque rapidly as the RPM increases and this can cause them to lose steps, where any of the servos, with the exception of closed loop steppers, have a nearly flat torque curve from 0 to their rated RPM. The closed loop steppers are kind of a special case, if they lose steps, the drive will attempt to correct the error. Where a more conventional servo drive will just keep adding more power to the motor until the torque limit is reached. At that point the encoder error will become too great or the internal drive electronics or controller will simply throw an error and shut the system down. Or the tool bit will just break, which is normally the case.
Under most conditions, the properly sized axis drives are operating at a small fraction of their rated torque while cutting on any machine. Where the maximum torque is used is in accelerating and decelerating especially during fast rapid moves. Take my lathe for instance, it uses every bit of the max available torque (29NM) of the servo to rapid the 1500lb+ Z axis carriage at 600 IPM over a 6 inch move. It requires a lot of torque to accelerate that much mass to that speed and get it stopped again in that short of a distance, it's frightening to watch it.