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When does something become an OS feature and when is it an add-on? Consider the use case. If you need to make a backup or restore data from one, by having this as part of the OS it is always available. It’s line having vi installed; it comes with every Linux distro, but a lot of folks use Emacs. It makes sense that this should be a system component.
Probably for the same reason Spanish used to consider ch, ll, and rr as a single character.