You answer your question yourself with
in fear of breaking something and having to reinstall (and losing something in the process)
Version upgrades are a big thing and unexpected things can happen. Would you be a happy user if the system auto updated to the next version and something truly broke? By forcing manual upgrade they can better inform users about possible unexpected things, and e.g. advice to do backups first. Thus users would be more prepared for this. Also version upgrades take a lot longer time than normal updates since it has update all packages, do other changes to the system etc.