The difference is that cars are mass-produced while airplanes are effectively bespoke.* And everything-on-the-touchscreen is cheaper to manufacture than physical knobs, so saving 25 cents per car matters. Airplanes are much more expensive than cars because they're all basically made by hand, so they can afford to spend more money on a better human interface.

I'm not suggesting touchscreens in cars are "a good thing", but I get the economics.

*Boeing makes around 700 aircraft per year. GM makes that many cars every hour.