Possible reasons of the present system
It might be very close to the point. With the remark that the "High definition" ATC tower and ground simulators have been available only for about 10-15 years.
And the fact that when using these simulators they still rely heavily on the use of expensive manpower. The flights that are simulated in tower simulations need to be run mainly by humans. Which is expensive. And that might be the reason limiting the use of twr simulations. The use of live traffic sdoes not cost any extra to the ANSP.
Also the simulated radio frequencies must be operated mainly by the same humans. On the other hand almost all facets of the simulation can be managed and produced by a computer with the flight simulators. And the ACC/APP/TWR radio frequency is simulated by the instructor in the flight simulators. And in many cases the instructor is the only human involved among the trained flight crew. (Except maybe for the technical personnel, that might have to be available anyway).
It is partly also a matter of fact that during these 10-15 years that "HiDef" tower simulators have been around only lately the use of simulations have started emerging in a way of simulating "true life heavy traffic".
One thing to bear in mind is, that tower environment is still so complicated that all the things that happen daily in a random fashion and take the attention of the trainee simply can not be simulated. Or at least that is the prevailing part of the belief systems at the moment, that explains why the simulators are not used as extensively as they arte in pilot training.
The other part could be that the 10-15 years of experience of simulators have not given the ANSP:s or the parties training air traffic controllers enough accumulated experience to use simulators effectively in a way that pilots are trained. The flight simulators have been around a longer time. It is only 20 years ago when tower trainees had their initial training with a "Real 3D" simulators: Miniature models of airplanes carried in the hands of other trainees that landed on a miniature airport.
Maybe there will be that one day that an ATCO trainee also steps to the real tower just for a couple of days of final validation check, when all the daily occurrances can and will be thoroughly simulated as in flight simulators. Just as pilots do.