Do they really need to launch a new series every year? Why not wait 2 or 3 years until they have gathered a good deal of innovative technologies? This let’s make a chip 10% faster and screen 20% larger and release a new watch mindset is Boring...
Because it's not always the same people buying the watch and some people want the latest tech in their new watch.
Once the whole thing is mature in 10 years, you'd get your wish.
When something works, you slowly evolve it, once and a while introducing revolutionary stuff, that's what they did with the Iphone.
By doing what they do they also keep the competition completely out of their market. They don't even leave them a chance to be competititive.
They also use previous year models as lower tiers in their range which means more and more people can buy them.