Why 'system' user cannot access /sdcard, follow-up question

Found the code and the answer to my question in case anyone else is
interested:

system/core/vold/volmgr.c

Processes are sent SIGTERM and then eventually SIGKILL if they don't
obey.

frameworks\base\core\java\android\os\Environment.java

private static void throwIfSystem() {
    if (Process.myUid() == Process.SYSTEM_UID) {
        Log.wtf(TAG, "Static storage paths aren't available from AID_SYSTEM", new Throwable());
    }
}