Ultimately the goal of a PhD is to learn to be a researcher and to produce a good new research result. If you can do that in three years why wait around another two or more for the sake of it?
I had a colleague who did a PhD in two years in Austria. In that time he got two top-tier papers published. If you’re getting multiple papers into top-tier venues then surely you’ve past the test? You obviously can do research and you obviously are producing good results as judged by a wide group of peers.
Why does the US drag it out so much?
I’ll tell you why - US PhD students also only spend about three years on their PhD. They spend the rest of their time doing masters-level taught classes, teaching (!) and working on their advisors’ projects instead of their own!
Or, perhaps there is an initial period of a year or so where the student is not yet doing a PhD, but is trying to produce results nevertheless. Saw that in Surrey, but can't remember how long students had after defending their PhD proposal and being promoted to PhD student.