Because nothing is. Because there ain't no such thing.
I don't know when people starting saying It's not rocket science to indicate that something isn't very difficult. I'm sure I never heard that back when I was an aerospace engineer* - or, as some people would say, a rocket scientist.
But there's never been any such thing as rocket science. Even when the field was being pioneered by Tsiolkovsky and Goddard, or for that matter by unknown Chinese experimenters many centuries earlier, what they were doing was a branch of engineering, not science. The cutting-edge work is not trivial. The people who do it aren't dummies. But they're also not scientists; by any reasonable definition of the two terms, they're engineers.
Maybe I'd be willing to go along with the silly usage if I thought it was just an acknowledgement of the complexity of the work involved. What keeps me from doing so is the figure of the rocket scientist in popular entertainment. The latest and most bizarre example is the character Henry on the TV show Eureka.
I loved the first season of the show (the second season approached dangerously close to suckitude), and Henry is an interesting character and the actor who plays him is outstanding and should be far better known. However, Henry, who told the hero, Jack Carter, that he used to be an engineer and worked on the space shuttle, knows everything about everything. He's the generic TV/movie scientist - the guy who knows every science the plot requires him to know and who can do all the hands-on stuff brilliantly, while solving monstrously difficult scientific problems in one hour. But of course he can! He worked on the space shuttle! He's a rocket scientist!
Why, Henry even understands quantum mechanics. Hmph. Let me tell you, despite having played a part in sending man to the moon and the Viking lander to Mars, I still think that quantum mechanics is smoke and mirrors and electrons are little gray ball-bearinglike things that have a definite position and velocity vector.*****
* Yes, I mentioned that as a way of pulling professional or maybe rhetorical rank. I didn't endure seven years in the aerospace biz for nothing.** If nothing else, it gives me the appearance of authority when I have this argument with people
** Okay, I also got a draft deferment*** out of it during the Viet Nam war, which is certainly far from nothing.
*** Until they changed the rules because they were getting so desperate for bodies to send into the quagmire, but by that time I was a daddy, just like Richard "The Dick" Cheney, but in my case it was a happy circumstance, not something we'd cynically planned solely (and soullessly) for that reason.****
**** But that's another story.
***** Oh, probably not. What do I know? I'm just a rocket scientist.