Because some organs rely on other organs to operate.
Explanation:
For an example, your entire body relies on your brain to give it orders. Your brain controls every part of your body, and without your brain you would most likely not be alive.
Another example would be your digestive systems. Your intestines rely on your stomach to break down your food (what's passed down to the intestines are referred t as waste).
An organ can be part of more than one organ system. For example, the ovaries produce hormones, which makes them a part of the endocrine system; the ovaries also make eggs, which makes them a part of the reproductive system as well.
