The is reason is simple: Arabic borrowed it from Hebrew ![Smiley :)](http://jtf.org/forum/Smileys/super/smiley.gif)
archaeologists would agree that hebrew dates back further than arabic..
in those days hebrew was written in the script ktav ivri, rather than the ktav ashuri that we use today.
I think arabic borrowed from hebrew(in ktav ivri) and aramaic..
I did once have a tree of the languages showing which developed from which..