Maxcast,
The difference is a source selector would need a stepper motor, which steps in defined increments. A pot just needs a normal motor, and you can get pots with built in motors, which makes it easy to add remote volume control.
For the problems with implementing a stepper motor, look at how much work John had to go through to add a remote volume control to his kit. He had to use a stepper motor for that application too.
So, it can be done, but could add more cost than people would think it's worth.
Randy