I don't see how anyone in the Apple forums can tell you why a product made by Microsoft doesn't do Arabic right. You will have to ask Microsoft.
As for iWork, nobody here knows why Apple has not fixed its Arabic bugs in the last 7 years either. But you can tell them you want this at
In the meantime, the best app to use is Mellel. TextEdit, Nisus Writer, and OpenOffice should work OK too.