There's no such thing as "written in unix". Unix is an operating system made by Bell Labs in the late 1960s. It's a bunch of programs that were written in the C and Assembler languages. The one program that runs everything is called the kernel. When the kernel is combined with a specific set of programs inside a file system that is organized to a particular standard, it becomes Unix or "unix-like".
Mac OS is based on BSD, which is Berkley University Science Department's Unix-like open source operating system. Linux is its own open source Unix-like operating system based on code that is not based on BSD. MacOS is not open source, but FreeBSD is. As a result, they are very different in capabilities, especially OpenGL (the 3D rendering abilities). While FreeBSD and Linux have binary compatibility packages that allow FreeBSD programs to work in Linux (and vice versa), that does not mean MacOS programs will work in either FreeBSD or Linux.
Why is that?
The simple answer is that Apple does not share the libraries of source code that many of their programs use, so it's not really possible to make binary compatibility packages for them for FreeBSD or Linux. I am sure Apple does this because those programs (like it's user interface) would work fine in FreeBSD and Linux, which would kinda sorta drop the retail value of their products, ya know.
That said, OpenGL for Linux and FreeBSD is solid. The worst part is poor drivers from both AMD and Nvidia. Current Nvidia drivers refuse verticle sync in Linux 64bit, even just browsing the internet, but they work great in Windows 7and MacOS, for instance.
As far as having PS2 in MacOS and not in FreeBSD or Linux, it boils down to proprietary drivers and MacOS's better use of OpenGL.