Well, that's not the whole story: consider another example, the various parts of Safari. Apple wrote that, Apple wrote the whole OS…should they have access to a kernel task port? Shouldn't I trust them to not do bad things? Of course I do, since I use the browser–but I am glad that those are split into separate processes and sandboxed, because an exploit in any of those instantly turns this access into a confused deputy problem. A confused deputy is trustworthy–but they're confused.

Adding exceptions means adding more points of failure, more complexities in code, more opportunities for attackers to bypass restrictions placed on them but not on OS services. Not only that, but you get the upside of having a unified model for Apple and your app developers "for free"–the latter which is of critical importance to Apple in particular, since they have had years of trouble in this area.