I'm assuming it's because nobody can just leave something alone. It's always gotta change, it's always gotta be made "better". And it probably generates a lot of marketing, good or bad.
If they leave it alone on what else would they be working on? Not on something in somebody's else department so it's either being layed off or convince the board that each year's iteration on the same things is the next groundbreaking invention.
You're describing the classical dichotomy between progressives and conservatives, a dichotomy which extends far beyond the political sphere to which is usually is applied. Whether it is in the arts, in architecture, in engineering, in design or in software development. UI design in particular seems to attract the type of person who is among the first to pull down Chesterton's fence [1] with no though given about what might be lost by this action.
