Why don't they just tell us what we can actually expect??
Because if they do that, they paint themselves into a corner. Endless people then sit around and fantasize what it will be like and build themselves into a frenzy of expectation that will inevitably not match what actually ships (sometimes better, sometimes worse) and then they'll be bitter and resentful. Take a look around these forums if you want to see that dynamic in effect. Another problem with pre-announcing too much is the "Sinclair Effect" -- it tends to make people less happy with what they've already got! Some people go "well I want planetary landings RIGHT NAO!" and throw their toys out of the pram in frustration because they can't wait for the new toy. Again, it breeds unhappiness in people with certain kinds of psychology. Lastly, there might be someone on the fence, who is thinking about buying the game, who says, "Oh, planetary landings in next release? I'll wait till then...." Sinclair was an early computer company that had a pretty well-selling product and pre-released that they were going to do something awesome next year and everyone stopped buying the current product, deciding to wait 'till next year. Sinclair went broke before they were able to finish the product everyone was waiting for.
Those are some reasons why.
Another reason is that it reduces their flexibility about deciding what features they can do and in what order when, which means that they can't have as much freedom to do "low hanging fruit" because now they're going to have a whinge-fest if they don't do exactly everything they said was on the next release's roadmap. Again, you can see this effect in play all over the forums. It's hard writing software for entertainment customers because they expect to be entertained and get excited about features. You don't get that with enterprise software (the only people whingeing about a specific feature in a specific release of Oracle are the small percentage of the user-base that need that specific feature) In entertainment software you're not dealing with a feature-list, you're
trampling on people's dreams and that's why the emotions often run so high here.