Why can Americans own guns?

Americans can own guns per the 2nd amendment on the Bill of Rights.

Though there are several versions and interpretations of the text, it basically says that it is "the right of the People to keep and bear arms and shall not be infringed."

It was made an amendment because the common people need a form of self-defense when a well regulated militia (the police) cannot always be everywhere at once and at every second of the day.