Because that's what NeXTStep did...
...and WindowMaker is the nearest thing to a complete implementation of GNUStep (the Free version of NeXTStep) that exists.
Personally, I think NeXTStep only looks nice when it's running on one of those dandy NeXT computers; it looks good from a distance but when you get up close it's very 1995. Of course, one of the problems is that there areally aren't all that many applications that run within the GNUStep framework; there are a few, but there's no GNUStep browser, for example[0]. However, one benefit of coding to GNUStep is that you can trivially modify your application to compile on OS X.
[0] Unless, of course, someone wrapped Gecko up in GNUStep when I wasn't looking.
Peter
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