Why was the ViewHolder class declared as a static class?

In general, we declare nested classes as static, when it has no dependency on the Outer class. In our case, the ViewHolder class has never referenced (accessed) any member variable from the Adapter class (Outer class), therefore we can declare it as static. To keep it simple a nested-static class is just another [Outer] class which is nested for readability, since its use is restricted to only its Outer class. Your nested class must be declared non-static (called Inner class) if it has to access the member variables of the Outer class.

I think ben instantiate the holder = new ViewHolder() so that it can be referenced in the setTag() method as argument cause I think its impossible to reference the ViewHolder class as the argument for the setTag()

Also why it set as static ? I think because we don't have to make the setter or getter method in that class to make changes of the member variable.