Why don't airlines board passengers from the back?

Some people claim that boarding from the back unbalances the plane as it waits on the runway. Planes seem like massive, powerful machines, but they actually require careful balance and calibration to fly. Thus, seating is carefully calibrated to maintain balance.Although parked planes tipping off-balance probably won’t hurt anyone, the experience is largely unpleasant and makes it difficult to align the door with the jetway properly and allow people to disembark and board.