Because if you did that, umpires would ignore the beep and call whatever the felt like. Seriously, these umps have huge egos (no disputing that).. that's why every ump has his own zone, some ump change the zone midgame to "teach the pitcher a lesson", etc. To implement your suggestion, there would need to be a shocking device in the ear piece to give them a good zap when they contradicted the computer
Just leave the homeplate ump at the plate to make the calls on the play at the plate.. The computer can flash the ball strike count after the pitch is delivered, so everyone knows the call. And IMO, it would speed up the game a little bit.
It would get rid of those times when a batter takes a 3-1 count, waits a few seconds, hears nothing , starts walking to 1b, only to hear the delayed strike call from the ump... No delayed call from the computer.