Comment by keyboardcaper

There have been POSIX-certified Linux variants. But the open source projects you use don't bother (for obvious reasons) and commercial derivatives like Android and ChromeOS don't need it. Similarly Window NT was POSIX-certified way back in the day yet its descendants aren't, even though they implement the same API set (via very different technology).

Because it's an expensive and lengthy process. And every version would need to be re-certified.

According to https://www.opengroup.org/openbrand/register/ macOS 15 is not certified.