The reason behind that is because swimming requires the usage of more muscle groups than most other sports. Also, those muscles are constantly moving.
In the water, you are also constantly moving against a resistive force, so your muscles are constantly working out.
The main idea of having a great cardio is not running in place for 30 minutes, because your body will hit a plateau and won't progress anymore. A really good cardio workout works when it trys to maximise efficiency, and to do this, you have to have variety in the exercises you do.