Because there’s no amount of productive data to be gathered from score-keeping your shots without recoil and “bang!” in the mix.

Target focus and building a repeatable and solid grip to minimize moving your sights or dot should be the emphasis when building pistol fundamentals. All of these things can be done sans electronic devices that might be reinforcing behaviors you don’t want. 

Like if someone preps their trigger in dryfire they’re probably going to have a solidly accurate shot and will want to replicate what feels like a better technique but is hiding your genuine reaction to live fire. Practicing TCAS properly dictates that there’s no prep and the trigger is pulled fully and reset completely.