Why one motor needs constant position feedback to work correctly — and the other is designed to run perfectly well without ever checking.
A stepper motor moves in discrete, fixed angular steps — send it one electrical pulse and the shaft rotates by exactly one step angle, commonly 1.8° per pulse. A servo motor, in the classic industrial-automation sense, pairs a DC or AC motor with an encoder (or resolver) and a dedicated servo drive that continuously measures where the shaft actually is. Both can position a load accurately. The difference that actually matters is what happens when something goes wrong — and whether anything is watching when it does.