Older cars are getting cut up if they suffer any sort of wreck damage, shorter ones are less desirable to shippers as rates are by the car load and smaller cars hold less. It takes less wreck damage to send a shorter car to the scrap heap. And yes, the bulk heads and center beams are structural members of the whole car so they can't easily be made over into a plain flat---besides, the demand for flats that short isn't that great these days either so it really would pay off even if it could be done. Right now there is a glut of center beams of all sizes.