What does that actually mean, though?
And "there's no reason you can't, if you wanted" is a far cry from "sure, the work is already done."
For example, something as simple as having a server-side rendered forum built with your favorite back-end language and now you want to build a client for it (web, iOS, Android).
That basically means massive duplication as you create a json interface boundary.
Yeah, you were already passing a { user, topic, posts } object to the template, so you can just serialize it your json api, right? But you can't because when you wrote that code, you knew the data never left the process. You didn't need to scrub the user.password_digest. Adding an api on top of an existing system is a lot of work no matter how much you hand wave about "well architected software."
But forget over the air network boundary. How about just wanting to implement the view layer of your Rails app with Go one day? How exactly would a "well architected" Rails app help you out?
The truth is that one reason that SSRs are simpler is because they are monoliths which spares you from certain classes of concerns. That's the trade-off you have to pay for if you want to cleave the monolith one day.