Why do photos taken with a camera's flash look so much better when the lights are turned off at night than during the day?

The premise of your question is not accurate enough. Not all photos are taken at night after turning off the lights and using the camera's flash to look much better than during the day. In some cases, the flash is used because of the light source. The brightness and illumination angle are easy to control, so it may be better than using natural light during the day. However, for experienced photographers, as long as conditions permit, it is better to take photos with natural light. The flash is only used as an auxiliary fill light when the natural light is insufficient. use.