um.. because in certain things mac osx servers are better.
http://www.apple.com/xserve/performance.html the powerpc g4 chip rocks. and you can squish 630 gigaflops and 20 terabytes into a single rack. a whole rack would put you at number 57 on the top 500 supercomputers list.
http://www.top500.org/list/2002/06/
equivalent to around 1000 intel based servers.