There is one specific reason why the subs aren’t a seperate file: formatting!
The subs are set up to look and feel like the Star Wars DVD releases so they come with a custom color and font. For this to work properly, for everyone, right away, and regardless of viewing platform, all this data needs to be embedded into the video file itself.
If you’re someone who isn’t a fan of how they’re presented, the formatting can also be overridden by internal player settings too.
Yes, you need to download the whole movie again BUT this solution gives the most artistic flair, viewer control AND its easier as a new viewer to just get a single file and have everything.