The very short answer is the type of person drawn to open source projects. Apple (I imagine) spends a ridiculous amount of time on user interface, paying experts to go over their UI and making sure it sparkles. Whereas the programmers who donate their time to open source are a lot more involved in their projects, and don't spend as much time on the "useless" end sparkle.

To be fair, this is a very broad statement, and thanks to projects like Ubuntu focusing more on user interaction it's starting to make headway, but even just looking at what a mark of pride it is to be using something non-intuitively difficult (vim, emacs, cli interfaces, hand-rolling operating systems and packages...), the realm of UI design on open source projects is vastly underpopulated.

As to Windows, that's anyone's guess. It may be they're just relaxed enough in their market superiority that they don't feel the need to bring as many designers to the table. Or it may be the other way around: Apple knows that one of the only things that sets it apart from other companies (and Microsoft in particular) is snazzy design, and they've chosen that niche to run with as far as they can.