Why is CPU often idle?

Your computer spends much of its time waiting for your input. In the case of this webpage, your CPU will do some minimal work processing the code of the page & such, but all this happens in a milisecond, while it will take you several seconds to read it.

Keep in mind that modern computers are almost never completely idle. Even when your computer is seemingly doing nothing there are processes in the background doing this and that to keep your OS going and ready for your inputs. These processes are hardly taxing, and so unless youre doing something particularly math-heavy your CPU is generally about 90% idle.