why dateTime is never null?

At first your question is not right, only reference type can be null or not null but dateTime is a struct means a value a Type , A value type is Auto initialized by the constructor of the class , so can never be null.

In c# the default Value for DateTime is midnight of 1/1/1 (yup, that’s the year 1 A.D)

If you want to make it nullable type then use ? symbol ex DateTime?