On Linux/FreeBSD it's typically the other way around; no matter how optimized and clean the back end is (relative to OS X), nobody seems to be able to put together a great working desktop standard UI supporting modern hardware.
Hopefully somebody will be able to provide a good free desktop on top good *ix platforms like Linux/FreeBSD etc, but all the people who held their breath died a long time ago.
The OS X packages like Homebrew and MacPorts make it very easy to run most of the standard backend stuff on OS X while developing, which means many developers prefers to work on modern proper functioning hardware while still deploying on standard Linux servers and similar.
In addition technologies like virtualization, Chef/Puppet/Docker etc support additional workflows which allow testing server work locally on OS X before deploying on Linux or similar.