Because when shooting TV series, the radio will inevitably receive noise and background sound. Especially outdoor shooting, when shooting in Hengdian, it is not used by a crew. Maybe you shot the blasting scene across the road in Where are you going, and it is inevitable to receive noise. Therefore, the general TV series are late dubbing, and there are two kinds of late dubbing, the actor himself and the dubbing actor. There are also some Hong Kong and Taiwan actors whose Putonghua is not standard, or whose voices don't match the characters in TV dramas. So most of the "soundtracks" you hear from mainland actors are re-arranged by yourself after the TV series is finished.