People often say "It's not rocket science" when they mean something is not as complex or challenging as it might seem. They use the image of rocket science as a metaphor for something very difficult to understand or do.

Here's a breakdown of why the phrase is used:

* Rocket science is indeed complex: Launching a rocket into space requires intricate knowledge of physics, engineering, and a vast array of other disciplines. It's a highly specialized and demanding field.

* Oversimplification for humor: The phrase is often used humorously to imply that the task at hand is relatively simple, even if it might seem daunting at first.

* Putting things into perspective: It's a way to reassure someone that they can handle a situation, even if it seems overwhelming.

In essence, saying "It's not rocket science" is a way of saying "Don't worry, it's not that hard."

However, it's important to remember that while rocket science might be a metaphor for complex tasks, many other things in life can be just as challenging and deserve respect.