I only dabble with OSX internals so it is not much more than an impression, but when I try to dig deep, I find that not much of the internals is on display. And I should say, by "exposing", I really mean "documenting" - clearly there are people who do know the internals well, they are just not sharing their "how to" knowledge much.
For example, OSX has the library com.apple.iokit.IO80211Family which provides the interface to its wifi hardware. It is completely undocumented. By contrast, Linux has the internals completely exposed because we have the liberally documented source and lots of open developer mailing lists, while MSDN has literally thousands of pages with documentation and developer discussions about hacking and developing drivers to fit into and make use of the Windows 802.11 interface.
For example, OSX has the library com.apple.iokit.IO80211Family which provides the interface to its wifi hardware. It is completely undocumented. By contrast, Linux has the internals completely exposed because we have the liberally documented source and lots of open developer mailing lists, while MSDN has literally thousands of pages with documentation and developer discussions about hacking and developing drivers to fit into and make use of the Windows 802.11 interface.