With physical parts, the development process is highly sequential. Pick the look, design how it fits, engineer what parts are used, manufacture tooling etc etc in a waterfall. If a revision needs to be made, the whole process needs to be re-started adding a huge amount of delays.
With a touchscreen, the physical touchscreen and the software that runs on it are parallel threads. You can make most UI changes without impacting the manufacturing/design pipeline at all. You don't even need to have planned what the interface looks like before you finalize the parts needed.