Here's why:

* Smooth muscle is found in the walls of internal organs like the stomach, intestines, bladder, uterus, and blood vessels. It's responsible for involuntary movements like digestion, blood pressure regulation, and childbirth.

* Skeletal muscle is attached to bones and is responsible for voluntary movements like walking, running, and lifting.

* Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body.

While skeletal and cardiac muscle are also important, they don't form organs in the same way that smooth muscle does.