None of those are from-scratch rewrites of existing software. They're mostly (not all) clones of existing software, with some (e.g. Linux) being more "inspired

None of those are from-scratch rewrites of existing software. They're mostly (not all) clones of existing software, with some (e.g. Linux) being more "inspired by", and some (e.g. Clang) being "We can do this better".