Rocket science seems simple today, by comparison of other more-complicated technologies. Using computer modeling and calculation, we can accurately predict probable trajectories in which an object will travel.

But it wasn't always this way. For a long time, we were trying to figure out how to measure the speed of gravity with blackboards and pocket-watches. Figuring out "who likes cake vs who likes pie" seems simple, yet we still wrangle with it.

There are similar idioms about brain surgery and brain surgeons. It's been around for a while, since the time when rockets did blow up with a bit more regularity. Now it's just a thing people say without considering whether it's an accurate expression or not.