The titles of King of Germany and King of Italy were closely tied to the office of Emperor itself, to the point of King of Germany almost being synonymous with HRE, and it would be a direct affront to the office of the Holy Roman Emperor for a vassal to usurp one of these titles.
Forming Germany or Italy while not Emperor should give the Emperor two options:
A. Be a bitch and let a vassal usurp one of your core titles. The new King stays in the empire, but you suffer a massive hit to prestige, imperial authority, and legitimacy. There should probably be some mechanic to encourage the election of a King of Germany as the next emperor, as it represents the real power in the Empire, and any Emperor who lets it go doesn't deserve any of his titles. Really, the only reason to choose this is if you are extremely weak and have no chance of taking on the upstart king.
B. Preserve your integrity and jettison the new king (but not his provinces) from the Empire. You still take a hit to imperial authority, but you can gain it back by reconquering imperial provinces from the renegade. However, said renegade should get a CB on you to claim/dismantle the imperial title, so that if you are significantly weaker than him, choice A. becomes reasonable. A King of Italy could also get the option to remove their provinces from the Empire after a time, as they aren't the heart of the HRE.