Well, it’s easy to locate the aircraft the same way you’d locate the cell phone, assuming the radio transmitter is transmitting. The fact that the airplane’s transmitters were turned off is the something that’s wrong in this world (in this case).
As one of my friends pointed out, being able to track cell phones is irrelevant since most airlines make you turn your phones off anyways (or at least put them in Airplane Mode, which turns off their radios), assuming the phones (and their transmitting radios) survived whatever happened to the rest of the plane anyways. I doubt even a Nokia 3310 would survive a drop from 30K ASL.
As I mentioned in the other thread, the transponder protocols have a finite amount of bandwidth. If every transponder were active at all times, their transmissions would interfere with each other and ATC would receive bad data. Or, the protocol could have been designed for much higher capacity but that would have had to have been traded off with some other cost: less frequent updates, more expensive equipment, or a later introduction date.
Additionally, the older modes supported IDs of only 12 bits (4 octal digits). With fewer than 4000 usable IDs after special reserved, there is a hard limit on how many transponders in an area can be uniquely identified (newer modes have upgraded to 24 bits with permanently assigned unique IDs).