Most people don't need a flashy new UI every year to stave off boredom - they want something easy to use and reliable. After two years navigating and interacting with the interface, it becomes second-nature.

This isn't to say refinements and improvements aren't welcome (which Apple has done with the iPhone), but a truly good design/UI will stand for years. (Great ones will last decades.) For another consumer-electronics based example, when was the last time Apple dramatically changed the UI on scroll-wheel iPods?