When it comes to English the problem can be split into two: the origin of the word, and its usage to refer to the planet.
The origin of the word is actually well known - English "earth" comes from Proto-Germanic *erþō "ground, soil", that in turn comes from Proto-Indo-European *h₁ér-teh₂. That *h₁ér- root pops up in plenty words referring to soil and land in IE languages; while that *-teh₂ nouns for states of being, so odds are that the word ultimately meant "the bare soil" or similar.
Now, the usage of the word for the planet gets trickier, since this metaphor - the whole/planet by the part/soil - pops up all the time. Even for non-Indo-European languages like:
- Basque - "Lurra" Earth is simply "lur" soil with a determiner
- Tatar - "Zemin" Earth, planet vs. "zemin" earth, soil
- Greenlandic - "nuna" for both
The furthest from that that I've seen was Nahuatl calling the planet "tlalticpactl" over the land - but even then that "tlal[li]" at the start is land, soil.
The metaphor is so popular, but so popular, that it becomes hard to track where it originated - because it likely originated multiple times. I wouldn't be surprised for example if English simply inherited it "as is", as German "Erde" behaves the same. The same applies to the Romance languages with Latin "Terra", they simply inherited the word with the double meaning and called it a day.
And as to why Earth has become the accepted term rather than ‘terra’, ‘orbis’ or some variant on ‘mundus’, well, that’s a tougher question to answer.
In English it's simply because "Earth" is its native word. Other languages typically don't use this word.
