Earth has many names actually, since it's a bit forthcoming to just go 'OKAY, THIS IS CALLED EARTH NOW EVERYONE ON THE PLANET, YOU KAY WITH THAT? OKAY'
Be it Terra, Gaia, Terre, Erde, Bhumi, Ulagam, Maa, Pamant, Fold, Ziemia, Zemlja, Zemya, Dine, Erz, Zewl, Jehun, Zamin, Tierra, Diqui, Chikyuu, Jigu, Bumi, Jorden, Bhoomi, Gi, Choma, Khthon, Dunia, Alem, Dunyaworld, Ardh', Ergir, Acun, Yeyuzu, Yerkure, Mundo, Daigdig, Lupa, Tatar, Zher or Qazaq. And that's just to name a few.
Fact is, only English speaking countries or international science teams refer to this planet as Earth. And as English is the language of choice for a lot of official shit in the world, they just call it Earth.
As Earth literally means the ground, mud, soil, and caves (Yeah, it's vague). I'd hazard a guess that the old English didn't intend for the word 'Earth' to mean the whole planet, but I guess it just turned out that way.