Hardware buttons and switches have to be designed, tested, re-designed, and validated very early in the process of designing a new model so that there is time to figure out how to manufacture / source all the parts, how they integrate with the rest of the car's systems, and how they'll be wired and assembled. Just imagine what the impact would be if late in the process a new feature needs to be added! Pretty much forget about it, add it in the next major model refresh.
With a touchscreen all those dependencies go away. The hardware team just says "there's going to be an iPad sized capacitive touch screen here for climate/infotainment, and another custom sized display here for the instrument cluster". The software guys can independently do the design of the UI, changing things down to the very last moment, or even after the last moment if the car can be updated.