Robert Bannister wrote:
what Apple calls "languages" are nothing more than sets of symbols and keyboard layouts.
That's not correct. When you switch the Preferred Language on OS X (at least to the one of the 34 currently supported), all the menus and dialogues of the system and apps change accordingly. To have IPA on this list would make no sense.
IPA is a specialized notation system that can be used by linguists to represent the sounds of any language, and if anywhere it belongs in the Input Source settings. Apple could in theory provide one of these for IPA, but it's not very practical, as every linguist probably wants a layout that reflects their native keyboard and the needs of the language they are currently transcribing.