The actual problem, from an app developer point of view is that you can't ensure that the lib behave exactly in the same way as on the developers system (and that may not just because of different versions but also because of different compiler settings etc). In many cases, the app developer also has some own patches inside some of the libraries to integrate them better into his application. It is often utopic to wait and hope to get those patches upstream.
Here's why you're wrong:
Unlike Linux, Mac OS X ships with a standard, stable, and exceptionally complete set of libraries. Apple goes to great lengths to maintain binary and API compatibility, and this means a few things:
- Most applications will only require Apple-supplied libraries, as there's very little that isn't provided by the OS libraries.
- Apple keeps the OS libraries up-to-date.
This feature-complete ABI/API-stable environment is not free for Apple to provide -- it takes a lot of effort to provide a consistent API across libraries and maintain the API/ABI compatibility across releases, but the advantages are tremendous. Mac OS X has been able to skate by without package management for years because this approach allows for drag-install drag-uninstall application distribution.
UNIX derivatives, on the other hand, were faced with a lack of standardization of core application libraries, a huge number of incompatible libraries to fill in the gaps, and a software distribution model that involves splatting files all over the disk. The end result is that you need a packaging system to maintain security updates, manage all the files on disk, and upgrade the world in lockstep due to API/ABI compatibility issues across libraries shared by wildly disparate applications.
So -- tl;dr -- it's not a security 'nightmare' because the core libraries that everyone uses are already shared and updated by the OS vendor.
This is also what makes Mac OS X so much nicer to develop and distribute applications for as compared to platforms that require a centralized package manager authority to keep things sane.
Something else that helps with Apple's way of doing things is their reasonably slow release cycle (when compared to most Linux distributions).