Apple can't distribute them - coreutils is GPLv3, which is incompatible with Apple's immutable system volume.
https://www.gnu.org/licenses/gpl-3.0.en.html#section6
(There are other issues with GPLv3 which make it a dubious choice for companies like Apple, but this one is particularly problematic.)
We also test GNU coreutils on macOS. It is a bit tricky since the other maintainers and I don't own any macOS machines for testing. But we seem to do well enough with a shared system that Adélie Linux kindly allows the compile farm [1] to use, along with some additional CI machines.