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...
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.