Okay, Google it... I don't care enough to google but thought it was a good question. Can we speculate instead of Google?
With more than 70% of the planet being water It'd be more logical to call it Water.
No real reason...it could've been named salami..
Earth was a word not already being used..
I thought of that...but doesn't every planet have a "ground" of a sort?
Well, just off the top of my head, I'd say The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (gē): the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, and later Norse mythology included Jörð, a giantess often given as the mother of Thor.
Originally, earth was written in lowercase, and from early Middle English, its definite sense as "the globe" was expressed as the earth. By early Modern English, many nouns were capitalized, and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets. House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (e.g. "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g. "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing? Does that help?
Ain't I intelligent?
If they're monitoring, they call it Idiocracy.
You get an "A."
That is noteworthy; and probably why we are so conventional...
...and what I said
It's called something else in almost every other language.
It's like why do we call water, water, in English but it's called eau in French.
Birth Place of Civilisatation?