Things might change...
In some countries the tower training is indeed done somehow like this.
The challenge at the moment is how to put incentive to
air navigation service providers or respective training organizations
to develop systems that can ease the step from the simulators
to the tower with live traffic.
The lacking incentive is partly due to the reason as Gonzo illustrates it,
that "HiDef" tower simulators cost real money and especially the needed amount of personnel to run the apparatus is so very expensive. Until technology matures, as Gonzo so well put it. And 360 sims have been around at least for 10-15 years. The training systems or training philosophies or belief systems associated to ATC training might need some upgrading.
So this leaves the cheapest (ANSP point of view, not necessarily from airline, i.e. customer, point of view) alternative to train the trainee with real and live traffic. Almost from scratch. With all the associated pain. (Not only customers...) And still, it is possible even now, to organize training just as it is done with pilots' type validation training.
It is partly a matter of incentive. Money in this case is just one incentive, although a highly prioritized one.