The problem with night mode is that it compresses the dynamic range of the audio which undermines one of the main benefits of having a home theater system. It's a poor compromise for bad audio mixing.
Most home theater systems are in a home where the noise floor is significantly higher than a theater, which the audio mix was targeting. Dynamic range compression is often appropriate and when applied correctly improves the experience in homes.
Manually turning the volume up and down is literally compressing the dynamic range; that's the goal here.