There are numerous reasons why developers/publishers announce games early, and it usually varies from case to case. For example, Bioware announced Dragon Age Inquisition long before they intended to because they wanted to reassure their audience and investors that the company was doing well despite the departure of co-founders Ray Muzyka and Greg Zeschuck. Then later while the game was developed, a decision was made to delay the game, and one of the bigger results of that were the return of race selection for the protagonist.
Don't get me wrong, in a perfect world where game development was simple and smooth, I'd love to hear about a game getting announced, and then subsequently released a few months afterwards. However, the reality is that game development is a lot more complex, plans change, and so on.
It's crazy, back when I was younger (before the masses started using the internet), the main source of gaming news came from monthly magazines like Electronic Gaming Monthly, GamePro, or Nintendo Power (Cheers to anyone else that was in the Nintendo Fun Club in the 80s! Damn I'm old). Nowadays, people can learn information as early as leaks on the internet (which sometimes force developers to announce early), and thus I'm seeing a lot of younger people used to having information as early as possible.
