Why does shunt generators produce only DC signal or emf?

This is in the wiring, the armature(s) field coils, etc, by the way if DC it would have permanent ( fixed) field magnets, not field coils. ac motors have Field coils, not to be confused with Field Amperage or Primary Coils- the latter being the (heart) of a transformer, consult Audels or similar technical shop manuals.