There's practical reason too. Translations made for overdubbing are designed to roughly take the same amount of time to say, and ideally to carry the punch in the same part of the sentence (so it fits with when the actor makes the corresponding funny face). This is not always trivial, to the point that sometimes translators change the meaning entirely (especially with eg comedy - entire jokes are reinvented because they were untranslateable so they just put a different joke in).

Subtitles are translated with brevity and ease of reading in mind. Translators typically take more freedom with nuances and emotion because, well, you're hearing the actor say it out loud in the original language and that's where you get the emotion from. So it's ok if the subtitle has it in a different order / different "punch", etc.

I'm also pretty sure that translated subtitles have as their primary target audience people watching the movie with the original audio. Taking the two step process of going from the source audio, to translated audio, to subtitles seems like a needless detour for that (both organizationally, but also qualitatively).