I suppose you refer to getSelectedItem()?
The reason is that if the combo box is editable, the selected item is not necessarily contained in the backing model and not constrained to the generic type. E.g. if you have an editable JComboBox<Integer> with the model [1, 2, 3], you can still type “foo” in the component and getSelectedItem() will return the String “foo” and not an object of type Integer.
If the combo box is not editable, you can always defer to cb.getItemAt(cb.getSelectedIndex()) to achieve type safety. If nothing is selected this will return null, which is the same behaviour as getSelectedItem().