Translating involves more than plain 1 on 1 conversion of words into their literal equivalent in a different language. After all, that does not necessarily turn it into a coherent, easy to read text, does it? Often much of the scope is lost by too literal a translation.
Ultimately, it is all about your message being the same in the target language as it is in the source language.
And about the translated text meeting your communication objectives.
In addition, there is also the matter of cultural differences which have to be taken into account when creating a text for a different audience.
A native speaker can best meet these demands. After all, the culture of my motherland and the subtle nuances in my mother tongue run through my veins.