As consumers, we might want this, but it's not a smart idea for companies. Their early announcements are tests to see if there's enough excitement long before the product is ready. If very few people get excited then very few people remember you never finished the thing because no one cared. Google, especially, is looking at these types of metrics early on.
This reminds me of a talk by Steve Yegge (an episode of the original old StackOverflow Podcast) where he says that the reverse is true a lot of the time. If you announce something and people find it lacklustre, they won't even care when you release the product even it is amazing.
Similarly if you hype before release and deliver, you will never be able to meet the hype that you set yourself and that will lead to failure.
