Because they don’t want you to upset the weight distribution.
Crews are schooled in the finer points of weight and balance calculations, but in practice it’s the planners, loaders and dispatchers who crunch the numbers. The boarding tallies are added to something called the BOW — basic operating weight — which is a book value of the ship itself, replete with all furnishings, supplies, and crew. (This BOW is adjusted time to time, such when components are removed or added.) Compounded with fuel and cargo, the result is the total gross “ramp” weight. Fuel used for taxiing is subtracted to reveal the takeoff weight.
Around the time of push-back or shortly thereafter, a detailed manifest is sent to the cockpit printer. It shows the passenger total, fuel and cargo totals, center of gravity information, and gives us all of the required takeoff speeds (V1, VR, V2) and flight control settings (how many degrees of flaps, the stabilizer trim setting, etc.), for each possible departure runway.