Because watches are bound around your wrist, thus making them much less likely to be dropped (and they're much lighter if they are dropped) - scratching is the

Because watches are bound around your wrist, thus making them much less likely to be dropped (and they're much lighter if they are dropped) - scratching is the big concern with watches. I believe many (most?) high-end non-smart watches already use sapphire instead of glass for that reason.