Why not? The same reason that motivates any for-profit company to do anything: Money. I don't know what the engineers at Beretta are thinking, but I can guess it has a lot to do with what the marketing department thinks can sell.
Even in the age of CNC machinery and polymer wonder frames, gearing up to produce a product that is substantially different from anything that currently exists requires a considerable amount of manpower, time which costs money. If Beretta thought they could make back their investment into Robocop's 93R (semi-auto for the states, of course), or Christian Bale's Beretta/Brick hybrid, I'm sure they would.
Then again, having a reputation for producing best-quality defensive pistols may be more important than putting out costume or collector's pieces. I don't know. E-mail Beretta.