As far as I know, camd.library was only used for MIDI. The Amiga by default, doesnt come with any midi port (unlike the Atari ST line). Most Amiga MIDI expansions took advantage of the serial port to implement it. So it was a reasonable choice not to include it in the OS.
In the case of realtime.library, what I understand, is that it is basically an abstraction layer to the system timers and alarm services. Most Amiga developers just banged the hardware instead of using this library, but it was put there to provide a standarised way to make use of these resources in a friendly OS way. A few sequencers, animation and video playback programs use this library.
So the desition to include realtime.library and not camd.library was in my point of view pretty reasonable after all.