Not just that, wiring it in to the single control bus is easier, otherwise you are stuck doing an analog to digital conversion anyways. Even new cars that have separate controls, these are mostly capacitive buttons or dials that simply send a fixed signal on the bus (so your dial will go all the way around, because it isn't actually the single volume control on the radio, but just a turn the volume up or down control).

Most of the cost savings is in having a single bus to wire up through the car, then everything needs a little computer in it to send on that bus...so a screen wins out.