Because your the only one asking for them? Seriously I have never, ever come accross a review or post stating "I give marks off for no volume buttons".
That said, congratulations! They do have volume buttons! You will find many mice out there offer at least 2 programmable mice buttons, and if they don't they probably register the tilting of the mouse wheel left and right as button inputs. These button inputs can be linked to keys and windows commands, 2 of them being volume up and volume down.
I don't recall how much control the default windows 7 mouse panel gives you on the programmable buttons, but many big players like logitech maintain their own software that will do this.