This is very basic explanation as I understand it (terms are not necessarily correct).
It isn't that there is no way to translate it, its just that it would be far easier and faster if the video and subtitles were separate instead of already merged. That's how most DVDs/Blu-Rays/Streaming services do videos and subtitles. You have video image with no text on it for a clean image. Then you add the subtitle track and get the text showing up over the clean image (or above or under or wherver you want). You can more easily access the subtitle track to get the text and then use a translator program and clean it up. Replace old subtitle track with new track and it plays over clean video and you get video with subititle in language you prefer.
If its hard coded as part of the video track then auto-translators don't have a text file to reference to translate. You would need to have someone who knows the language manually translate it as opposed to someone editing a MTL so it makes sense. Then you also have to deal with cleaning the video itself (i.e. removing or covering the old subtitles to replace with the new). This requires going frame for frame through the video and removing the old stuff and possibly redrawing what's missing (looks best and what lots of webcomic translators do), or just slap like a black box and the new text over the old text (but that looks really bad).
So in summary, not overly difficult to do, but just very time consuming, especially if they want it to look good. As for someone trying, I genuinely hope someone does. Just know it would take a while if someone decided to pick it up.