Anecdotally, I've visited a group enthusiastically exploring the possibilities of the then-new CJK Unified Ideograph Extension B in Tokyo over a decade ago. The CJK issue mentioned here was a bigger problem then than now (most developers know how to handle it properly, e.g. with language declarations in HTML etc.).
People will use legacy encodings. That's just a fact of life. Sometimes out of inertia (i.e., it just works, so why bother changing it?), sometimes out of technical limitations of their pipeline, sometimes because of a lack of understanding, rarely out of principle. I'm stuck fixing an issue with some numb skull sending us SAML requests declared as UTF-8 containing some obsolete encoding instead, and this is for Dutch text! This just happens.
There is also nothing wrong with text in Shift-JIS, if that encoding encompasses all characters used.
If you change your PC or your phone locale to Japanese and access mostly Japanese website, the font display on most major OS works well. It's probably fair to assume that's a very common setup for Japanese users, and probably for Chinese and Korean users as well.
I think the CJK "problem" is more apparent for international users who don't have the locale setup or dealing with multiple languages plain text at the same time.
[0] https://w3techs.com/technologies/segmentation/tld-jp-/charac...
I don't really really know, but this random yahoo answers [0] page seems to agree with me that really you're gonna use UTF-8 for new stuff in general
One point that answer brings up that I hadn't thought of: older flipphones would have good shift-JIS support but not necessarily good unicode support
My guess is that "text as images" might partly be that, partly what you are saying in a legacy sense, and also partly for the same reason old sites in the US would do that: to make something look the way you want and you already have a photoshop render [0]: https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q14...