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).

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

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