Knowing the words of a language isn't the same as truly understanding it. Idiomatic speech, regional dialects, slang and subtle differences in connotation flow naturally for native speakers. For translators and transcriptionists working in a second or third language, many of the nuances of the language are lost.
Intuitive knowledge of which word to use comes only with decades of experience with a language. Mistakes in translation can be unintentionally grotesque or amusing - not emotions that businesses or scholars want to provoke in readers. The English words "corpse" and "body" overlap, but aren't always synonymous. When a native English speaker watches a police drama on television, the actors use the terms interchangeably. However, a native speaker would never refer to body lotion as "corpse lotion," a mistake that a non-native translator could easily make. (And I’ve seen this before no joke.)
Baseball has given the English language a host of idiomatic phrases such as "cover all the bases" or "ballpark figure" that would baffle a translator who had little knowledge of the sport. Idiomatic speech is so closely identified with native speakers that soldiers use idioms as passwords to identify friendly troops returning to base. The same cultural quirks and idioms leaven other languages, too. A Spanish translation service that transcribed the phrase "mantenerse en sus trece" as a directive to "maintain your thirteens" would be literally correct, but would lose all the sense of the phrase's connotation of "sticking to one's guns."
Regional inflections make language even more complex for the non-native translator. While a Mandarin translation will reach most Chinese readers, text intended for Shanghainese readers should incorporate the related, but different Wu language, not Mandarin Chinese. English readers recognize these regional differences when they compare text written for an American audience with that prepared for readers in the United Kingdom. To an American, an article involving boots and bonnets sounds like a fashion article; to an English reader, it's clearly referring to cars.
Native speakers who translate from a second language to their mother tongue go beyond the word-for-word transcription that translation services offer. They're able to glean the meaning from the words as well as the dictionary definitions and choose the most natural options in their native language.