I think the key point is that while the Contacts and Calendar applications are typically pre-installed on an Android device, they are still applications and not a part of the operating system itself. Any third-party developer could create a competing application that would have equal status on a device, while supporting CalDAV and CardDAV as well.
Trying to build CalDAV and WebDAV into the Android OS wouldn't work, because they would be dependent upon those applications, which would then make them essential OS applications.
Contacs ("People") and Calendar are part of AOSP, althrough most vendors do not ship them.
It does not matter, as neither AOSP nor vendor-specific Contacts or Calendar applications contain any protocol code. These frontends use contact and calendar specific content providers, which in turn provide plugin based API for sync adapters. Google's own sync adapter is such plugin, as well as Facebook's or even Microsoft's (for Skype contacts).
It is true, that there are no CalDAV/CardDAV sync adapters on the devices. However, because not everyone is interested in one, it is perfectly fine to install them from Play Store for those interested.
Trying to build CalDAV and WebDAV into the Android OS wouldn't work, because they would be dependent upon those applications, which would then make them essential OS applications.