Please note that using a CD or preupgrade tool does not mean a complete reinstallation. It just means that the upgrade should not be done from a running system but by booting a known stable and non-changing system to do the upgrade. It's not as convenient, but still not that bad a situation. At least it allows major changes to the system without endangering the update system itself. For example it's kinda bad if the update removes an obsolete compression library before installing its replacement leaving the system unable to read the package containing said replacement.
It's certainly not impossible to avoid such problems in an on the fly upgrade, but it takes quite some effort. Effort which may be seen as better invested in other features like a solid unattended installation or upgrade process.
Same was true for openSUSE btw. On the fly upgrades tended to work but have only been recommended for the last couple of versions.