We tend to be USA centric in our thinking. In many parts of Europe, riders stay with their clubs doing normal rides in non-PBP years and randonneuring is indeed focused on qualifying for PBPor sometimes others for some . Indeed, many French riders were initially cut out in 2019 because they did not have the 2019 distance to pre-register. In 2023, I read that they are more generous with quotas for French riders (as they should). A SR series can be thought of a preparation for a 1200K or a significant accomplishment in its own right. I have too much time on my hands right now and was looking at the brevets that DNF riders did leading to 2019 PBP. Three things stood out (sorry, not scientific). 1. Early Series without a lot of brevets afterwards. 2. Flat course 3. Inexperience (higher rusa numbers). Warmer climate riders seemed to drop out more, too.
There are probably a thousand reasons randos like randonneuring. THAT might be a good thread.
BTW....I always felt the jump from 200 to 300k on a hilly route was a real eyeopener for newbie randos.