Personally, the times I upgraded a FreeBSD box the process felt very smooth. To the point that I’m trying to understand @rsyncs point and seem to be missing something critical.
But, darkhelmet and I were discussing updating FreeBSD at Yahoo; doing the update from the Handbook wouldn't be the right way to do it, because you would miss out on Yahoo specific patches, which were likely required for some key Yahoo software. I don't think it was common knowledge how to do it, so some documentation might have been nice, but mostly teams at Yahoo just didn't do it at that time for better or worse.
FreBSD also supports in-place upgrades for major version numbers:
* https://docs.freebsd.org/en/books/handbook/cutting-edge/
* https://www.cyberciti.biz/open-source/freebsd-13-released-ho...
There could be policy reasons why someone may decent not to do it: if you have a large fleet of cattle machines it may be easier to just redeploy from scratch. Home(lab) users and pet machines at companies may do otherwise.