I feel like there are several factors contributing to the final picture.
First, most of the Japanese models start as college students, so in a sense it can be also seen as natural that they leave the industry once they graduate and get regular jobs.
Being a porn actor is also not that rentable because there are only as many scenes (specially gay oriented ones) you can do on a regular basis. Even more famous faces who have/had a career built around sex such as Koh and Hiroya would do a lot of work other than actual porn shooting, like modeling, gogo boying, working at bars and etc. Once they get other gigs going well, it's understandable the priority for porn shooting goes down, unless the model is specially interested in doing porn. In that sense I think American porn is able to handle longer careers also simply because there is more market (they sell to the whole world while Japan sells most domestically), so there are more scenes to be shoot.
I also do think there is historically an emphasis on always having new faces on Japanese gay porn industry. If anything that emphasis is actually going down since the 2010s and we are getting longer careers. In older Japanese gay porn most models don't have even a stage name and most works were clearly focused on selling this concept of "a regular straight guy who agreed to appear in this video" (which I do think was true in most cases, to be honest), and the regular appearing models would be deliberately covered (hence the goggle guy concept), since that's not what was being sold.
And finally, the model feeds itself, of course. As the discussion in this post clearly shows, Japanese porn companies already work with a business model that's based on short-lived careers. For series like "Extreme Interview", "Cock delivery" and "Virtual Date" to keep going on year after year, it's clear there is a need for new models to come in and old models to leave. (Unless you record literally the same scene for years non-stop, like
someone we all know)
Since most works produced by the main companies are based on regular series, after a while the models simply run out of scenes to shoot. If most works were independent produced stories (such as, let's say, Men.com does) there would be more room for models to stay for longer recording new scenes.
The difference of the business model can be clearly seen when you check BoysLab, for example. While they do have series, those are just very loose themes such as "romantic" or "bitter ending", instead of copy-and-paste scenes such as Extreme Interview. Each "Sweets" or "Style Select" scene is an independent work, meaning they can keep doing more "Sweets" scenes with the same models over and over without much worry.
