@Alpha-Admiral Germany goes before Japan, so if you attack USA with germany, the benefits of waiting with Japan (low US income) are a bit scrambled. Also, you may not be able to kill the entire UK fleet without sacrificing the precious German battleship if you divert subs to other objectives.
The diplomatic rules in Global 40 give you precious time and space for the Axis to play and cavort all over the UK, France and China. You need to sacrifice that advantage at a maximum cost to the Allies.