gcc/clang are completely open source. In fact, the ones online are far more recent versions than what apple has been shipping. I believe apple still ships gcc42 as of the last free xcode.
That's because GCC has changed licensing to GPLv3, and Apple didn't like that. So they left with last GPLv2 version.
Are there really that many compelling additions to gcc since then? This is a serious question, not a rhetorical one.
the optimizers are significantly better, enhanced profiling, standards compliance more strict, c++0x additions, and improved compile times.