Isn't one of the reasons for handshakes thought to be to show that one isn't carrying a weapon? Since women aren't expected under most circumstances to go about armed, they have less reason to shake.
I think, as modern humans, we try to retrofit these reasons on to old customs but I have a sense that a handshake is simply the most convenient expression to signify to a person that you have an inherent respect for them or recognise their value.
