IMHO, there are way too many variables from event to event to establish a TDA rule. In addition to the items K-Lo stated, what about events like the WSOP where there are hundreds of tables in play in different rooms, etc. What do you do if the sound system isn't working in one room, so the TD's announcement isn't heard?
The solution, in my view, is to establish a "House Rule" considering the circumstances at a particular event. Number of tables, number of locations, is the "official" clock visible from all locations, etc., etc., etc.
In the Native American casino closest to me, they have a clock that is displayed on one of the TV's. However, the dealers have to rely on the audio announcements because the way the room is set up every dealer has their back to the wall and cannot see the TV. Move the TV you say, HA HA HA.
First of all they would have to buy another one and second there is no wall on the other two sides, only a waist high partition separating the Poker Room from the rest of the casino. Finally the most applicable reason of all, it would cost money and they aren't going to spend one damn dime for something like that.