The reason is that there are tradeoffs. Fixing the timestep guarantees a frame of latency, but most (all?) integration methods used to simulate physics break down with wildly variable timesteps.
Tying simulation to framerate is often the best choice, when you consider that most hardware is average, it minimizes latency for the average user, and results with a more accurate simulation, so long as people keep vsync on (why do you need framerate greater than your monitor's refresh rate anyway...).
Tying simulation to framerate is often the best choice, when you consider that most hardware is average, it minimizes latency for the average user, and results with a more accurate simulation, so long as people keep vsync on (why do you need framerate greater than your monitor's refresh rate anyway...).