The air conditioning pack produces the requested temperature, the ACM is just a component of the pack. The ACM is a turbine/compressor. Some hot air bypasses the ACM, some hot air enters the compressor, is cooled by an air to air heat exchanger, then is expanded through the turbine thus cooling it.
All air that goes through the ACM is cooled. The only way to vary it as you suggest would be to slow the turbine down; more wear, parts breaking, unnecessary complexity. Simpler just to bypass some hot air.
To give you an example, if you selected full hot on a system that routed all air through the ACM, you would need to stop the ACM turning to ensure no cooling took place.