Specifically wiring harnesses and installing them are expensive. Touchscreens require less wires, since less buttons means less wires, which saves amount of money/time, and % chance of defect, per car.
What? There need not be wires for each individual button... they could all be mounted on a single PCB that dumps activity to a CAN bus.
Who cares. It doesn’t matter. Touchscreens in cars suck.
A button means: a pin in a microcontroller or in a multiplexer, a pin on the connector, enbironmental tests (water splash), ESD tests, etc. All those cost development money.
In brief, a touchscreen is meaningless and doesn't work until you show something on it. When you show something, that something will cost development money.