The word jungle comes from the Hindu word "jangala," That refers to any location not inhabited by humans. Before they were called King of the jungle they were called King of the beast.
All cats think they're "kings". Lions are big cats. Saying, "King of the plains" or "King of the grasslands" doesn't sound too regal, whereas "king of the jungle" has a certain ferociousness to it. Most lions prefer to be called "King of beasts", as that eliminates any doubt of their feline greatness. {note: Since jungles usually are densely shaded areas, and cats like sun, no lion in its right mind would even go there - despite their bragging.)
Just because. Doctors don't have to be in a hospital to be a doctor. Criminals don't have to be in jail to be criminals. The Queen of England is still the Queen of England wherever she may be.
In elementary school, my son was the narrator for a play about animals. And they had several lines that he simply refused to say because they were wrong. One of them was "in the jungle the mighty jungle the lion sleeps tonight." Teacher literally had to rewrite because of it.
Echooos 