Such keys were in the original Xerox Star keyboard in 1981:
(from http://www.digibarn.com/friends/curbow/star/keyboard/ where you can find more pictures).
I don’t know why they didn’t make it into the later hardware though. Incidentally, the removal of these keys has contributed in making keyboards very unbalanced with regard to use of hands: the right hand does most of the work. Cf. http://www.yorku.ca/mack/CHI01b.htm