Why? Because they give zero communication. That is the problem. It is called managing expectations, something companies generally try to do. So yes, it is on them to do so. Even in my own small business managing client's expectations is something I have to be aware of or deal with complaints later that are sometimes very silly.Why speculate and create a self-imposed expectation?
As to ruining surprise, is it really any less of a surprise doe them to tell us a month ahead compared to a week ahead? Any other game I play it is known what is coming months ahead of time and generates a lot of hype and buzz. I think the reveal at any point would be a surprise and therefore good. Also my example of announcing Australia was just that, an example. Could be "we are working at diving animations" or "flying birds is something that is being worked on for the future". Literally anything would be better than their current stance of complete silence.
Sure there are people that will not like it, you cannot avoid that by being silent. People have not liked the last two dlcs too.