The general layout of the standard keyboard was designed before the introduction of the mouse.
Since number entry was one of the primary functions of the keyboard at that time, having the numeric keypad on the right side made sense.
Nowadays, most users use the mouse far more than the numeric keypad, so the keypad now occupies the space where the mouse should be. The Evoluent Mouse-Friendly Keyboard has a more sensible layout with the numeric keypad on the left so your mouse may be much closer on the right.(Patents pending.)