It's unclear whether the LLVM-style implicit copyright assignment is really enforceable, and this certainly isn't a forum to debate it. In any case, it doesn't really matter, because the only reason copyright needs to be assigned (AFAIK) is to change the license.
This is not the only reason (and in the GNU projects case, not a reason at all), the main reason is to be able to enforce the copyright of the work without having to call in everyone into court. If only parts of GCC where copyrighted by the FSF, then the FSF could only sue only for those parts.