IAAF rule 162.2 ("The start") says:
All races shall normally be started by the report of the Starter’s gun held upwards.
and 162.6 ("False start") specifies that:
An athlete, after assuming a full and final starting position, shall not commence his start until after receiving the report of the gun.
Your question, I think, proposes changing this rule, and you're looking for arguments against. The IAAF rules don't offer justifications, so it's difficult to argue from grounds more specific than "but that's the rules", but here are two reasons a mechanical system would not work:
- Set: Races where athletes are likely to gain an advantage by anticipating the start are also in the races which include a "set" stage in the start sequence ("To your marks," "set", BANG). The rules provide for the starter verifying that all athletes are in the proper set position (also defined in the rules) before sounding the gun. They could do this before allowing a mechanical process to take over, but because the time between "set" and the start is the main unpredictable variable for starters, this probably wouldn't help much. In other words, to make the start truly predictable, we would need to remove the requirement that the starter verify all runners are in the "set" position before starting.
- Jumping the gun: The idea that the start should be predictable changes the actual race being timed. As the rules currently stand, the race starts with the athlete hearing the starting sound and ends when their torso breaks the plane of the finish line. As such, the athlete's ability to react to the starting sound efficiently and effectively is part of the event as it stands. By making the start predictable, you change the beginning of the race to being from the starting signal, and you allow the athlete to anticipate the signal (as long as they don't actually move). (I hope I've explained that change clearly enough.) This changes the race significantly on paper even if it seems like a slight change in theory.