I don't think it's an optimisation, but a design choice of simplicity; in fact, CP/M (and DOS) would parse the first two arguments if present, and put the results into the filename fields of the two default FCBs, allowing programs which take two filename arguments (traditionally, an input and an output) to be implemented easily without having any commandline parsing logic of their own. More info on that here:
It would be interesting to trace the origins of command-line parameter passing design even further, to the mainframe OSes that came before UNIX, but I'm not so familiar with them.
