There's two hurdles with that. One is UA releases, and the other is internet leaks.
The UA testing process means that even when we don't know exactly what books are coming, we often have a really good idea. See: Apocalyptic Subclasses and Dark Sun. So people are going to make lists and set expectations, and by making official announcements WotC can at least try to set the expectations at a reasonable and accurate point.
Internet leaks are a sad fact of life these days. Half the video game big reveals get spoiled ahead of time because there's just so many people involved at so many levels that's it's super easy for something to slip out ahead of time, intentionally or otherwise. So again, by making an official announcement WotC can do it in a controlled way and with as accurate and up to date info as they have. Which is better than people forming half-assed opinions based on fragmentary leaks.
We're in the Information Age now, and that has its downsides. Trying to lock down any hint of the books until two months before release is a futile goal. Better to put out the information yourself, so that it's accurate and complete. Because incomplete rumors and misplaced expectations are even more damaging.