Is GTK really a benefit compared to Cocoa?

Uh, What? Mainstream hardware support from OSX?

Not only does it have poor support for common hardware, it has artificially restricted support. What it does support, it supports reasonably well, but that hardly qualifies OSX as a system that supports mainstream hardware.

OS X does provide most of the benefits of Linux, except for large portions of the OS being closed source.

You get a terminal with bash or whatever shell you wish, which works great (none of the Cygwin crap). You can compile most non-GUI Linux apps for OS X (and many GUI ones). Etc.

I rarely run into things that I can do on Linux that I can't do on OS X.

Native GTK is not the Apple problem. GTK developers could use Mac application paradigms instead of Windows's and embrace GNUStep/Objective C for GIMP and GNOME instead of implementing their own toolkit. They made the choice of going Windows-like way, not Apple. That's the reason of why "on Windows for years, but still tricky on MacOSX".

Volunteer time/interest.. not enough people want to scratch the itch.

The need for multiple binaries - (OS X 10.4/10.5) x (PowerPC/Intel) x (32/64 bit, maybe) - is also a pain. You can cross build, and you can munge multiple builds into a "universal binary", but it's not obvious enough to avoid the same discussions happening on every open source project's mailing list.