How to use Java Static Method [on hold]

Static method we can access using Class name and new instantiated variable name(new object). So my question is why don't we declare all methods as static?

Class ShoppingItem{
public static void addItem(Item item){
        }
}
Class TestShoppingItem{
ShoppingItem.addItem(Item item);
ShoppingItem sItem = new ShoppingItem();
sItem.addItem(Item item);
}