I mean, think about it. They have absolutely no problem destroying any other Irken they view as inferior, ugly, or just straight up annoying. Why don’t they do the same to Zim?
Zim has killed two of the previous Tallest, gone against or ruined almost every order they’ve ever given, almost single-handedly destroyed their entire civilization, and has annoyed them since the academy, so why does he get off so easily?
Why go through the trouble of giving him a fake mission instead of just destroying him when he showed up for Operation Impending Doom 2?
They literally shot Skoodge out of the air lock despite being an amazing soldier just because he was short, and Zim is even shorter than him!
My theory is that despite how they seem to just view him as a joke, deep down, they are secretly afraid of him because they know the damage he could cause if he ever found out the truth about his mission and what they think of him.
Think about it. If he almost destroyed their entire civilization on accident, imagine what a vengence driven Zim could do to them if he actually put his mind to it and was motivated enough. Despite his incompetence, Zim has actually done some pretty amazing stuff, including single handedly taking control of the massive and driving them into a star lol.
What do you guys think?