Because translation isn't a one-to-one transformation. A bilingual speaker doesn't have a single abstract thought that they are then able to express in two diff

Because translation isn't a one-to-one transformation. A bilingual speaker doesn't have a single abstract thought that they are then able to express in two different languages. Rather the expressions themselves represent similar thoughts that are grounded in the overall cultural context surrounding the language. Translation attempts to minimize the difference these expressions have on their respective audiences, but doing so requires understanding the text like the target audience. Another way to think about it is that translation is not a question of "Express this English sentence in Chinese", but rather "Rewrite this sentence in the style of (some specific Chinese person)". You'll get a very different result if the target is "physics professors from Beijing" versus "rice farmers from Yunnan".