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.
Posix is more an art form documenting the far gone past rather than a practical tool
But the only concrete example I can think of is that `cat -n a b`, if `a` doesn't end in a newline, produces something that is obviously not what the user wanted. GNU cat does it right.
I didn't report it to the openbsd people because I didn't want them to yell at me that `-n` is nonstandard anyway with no "right" behavior, so I should STFU. Which would be what I'd expect from having reported problems in the past.