Question

why does the duration of day and night keep changes

Answer 1

Because of earth's rotation the duration of day and night changes

Answer 2

The overall change in duration of day or night is entirely due to the rotation of earth on its own axis.

This is a phenomenon and the overall time or duration in terms of hours of daylight is dependent upon the earth which is further tilted and it is not completely straight in its position.

So if we consider an individual to be at one point of place at a time, then the duration and the overall respect of day and night duration varies accordingly.

So the two important factors are the time of the year when when we consider evaluation and the place from where we start to evaluate.