Workout mode resistance control on/off selection

When using workout mode can an additional toggle switch like the ERG mode on/off be added to allow the trainer to be controlled by the hills as in a normal ride.  Why is it that we must be controlled by Zwift to say we can only have a ERG mode on or off and if off then it is a “flat” resistance…makes the ride less life like…I could just be using a “dumb” trainer and get the same effect.  I thought Zwift was trying to bring the outdoors inside?