Why doesn't Google announce Google Chromebook updates?

With Chromebooks and the Chrome OS still being relatively new I do not think that Google always has a clear plan on updates. As they find new bugs they may change the update schedule. I think that is why they don't announce them.

Hemlock, do the updates just get force downloaded to the machine like Windows auto updates?

Yeah, the Chromebook does get automatic updates. It is weird because you can also click the wrench icons to check for updates. Sometimes when you press it there will be updates that didn't get automatically updated like it is supposed to.

I imagine there are some updates that they choose to make manual because it could make changes to saved data?

I think modder is right. It seems like people were pretty upset about Chromebooks having automatic updates so I think this could have been Google's way of compromising.