Loydd,
You have to remember that what is in your transmission is mechanical gearing in a lubricating fluid, so a brake that is mechanical would involve a pin or a cog that latches. On a 3500lb car that may work very well within the confines of an automatic transmission but our transmissions move upwards of 60,000lbs or more and that kind of load locking would not necessarily be a small somewhat simple locking device. Given the vast majority of the heavy duty vehicles that are mated to this transmission have full air brakes with spring driven parking/emergency brakes there was little need in the design to handle that parking brake issue. While it may be possible, that kind of load parked on a hill with tension on whatever mechanical lock is involved, could be impossible to get to release also.
Later Ed