It's just not needed and gets in the way. In the original inotify library it was most likely created to avoid dependency on the Unix library, which in this case is not a problem.
Actual errno value is also not always equal between platforms. With Unix library we always get the correct error code. In the original library this was not a concern because only Linux was targeted. inotify-compatible interfaces are now available on BSDs, Darwin (macOS) and (some) illumos-based OS.
It's just not needed and gets in the way. In the original inotify library it was most likely created to avoid dependency on the Unix library, which in this case is not a problem.
Actual errno value is also not always equal between platforms. With Unix library we always get the correct error code. In the original library this was not a concern because only Linux was targeted. inotify-compatible interfaces are now available on BSDs, Darwin (macOS) and (some) illumos-based OS.