the DCS is programmed with all the relevant data for that particular aircraft, and therefore will plan its most economical trim, based on all things planned/booked for the flight
There's the answer - keeping us packed like sardines makes for a more economical flight and we save some money for the operator
Seriously - I seem to remember getting on half empty BAE-ATP flights and having to sit in a crowd at the back to help the weight and balance. Anyone else done this?