The big trouble is that the torque required to turn a bolt (or nut) a specific distance onto a mating thread is somewhat variable. Factors such as cleanliness of thread, smoothness of the metal surface, the metal(s) themselves (try putting a Ti nut onto a Ti bolt to fully understand the meaning of the word "binding"), surface tratment, etc. all can have a huge effect.
Don't believe me? Try the following experiment:
Take any nut and bolt, assemble them "dry" around some washers to a given torque, counting the turns you have tightened. Undo the assembly and put some grease onto the threads (any grease, or oil, will do) then repeat the assembly to the same torque. You will find that the nut winds on somewhere between 1/8 and 1/2 a turn further for the same applied torque.