Ah, well, the content has to fit localized ads! That's it!
(You don't want that distracting break in languages between revenue earning productions and embedding media fluff. I guess, in this context, the plug-in is as evil as ad-blockers are.)
In fact, there must be an incentive here for Google to auto-translate a video only when it can sell an ad in the translated language for more than one in the original.
I suppose we should be glad that they've not (yet!) gone that far, or it would become even more of an unpredictable mess from the end user's point of view.
Can't wait for consuming content in its original form becoming a violation of the ToS! :-)
(I can see a bright future where smart TVs come with mandatory, non-optional instant translation… and maybe, if we were allowed to dream, with realtime character replacement and background adaptation to fit the next ad context. It's the logical step forwards from colorized B/W… /s)
Also, advertisers get that option, because they're the only users YT cares about.