Games are frequently announced when they are little more than a concept and a target. The reasons have more to do with running a studio than with selling a product.
Recruitment depends on the project being known
Large productions need specialists who are usually employed elsewhere, and persuading them to move requires telling them what they would be working on.
Confidential projects can only be described in vague terms, which is a weak offer against a competitor who can name the game and the role.
An announcement converts a secret into a recruitment asset, and studios often reveal a project at exactly the point when hiring becomes the constraint.
Platforms and partners want the reveal too
Console holders and storefronts build their own showcases around upcoming titles, and those events need content scheduled months ahead.
A studio that has taken platform funding is usually contracted to appear, so the reveal date is negotiated rather than chosen freely.
The same applies to engine partners, hardware vendors and subscription services, each of which has a calendar the announcement has to fit into.
Audience building takes longer than a marketing campaign
Communities form slowly. A mailing list, a wishlist count and an active discussion take years to grow to a size that affects launch performance.
Wishlists in particular function as a visibility mechanism on storefronts, and accumulating them requires the page to exist long before the game does.
A campaign that starts a month before release has no time to build any of that, so it can only reach people who are already looking.
Early reveals carry real costs
A game shown in a concept state sets expectations against footage that may not represent the finished product, and the gap becomes a story at launch.
Long gaps also invite fatigue, where an audience that was excited at announcement has moved on by the time the game is available.
Development can change direction, and features shown early sometimes do not survive, which is a communication problem the studio then owns for years.
The industry has shifted towards shorter windows
Many publishers now reveal closer to release, sometimes only months ahead, having concluded that a shorter campaign concentrates attention rather than dissipating it.
That approach requires a game far enough along to show honestly, which is only possible when the schedule is stable and funding is already secured.
Studios without those advantages still announce early, because the announcement is doing work that has nothing to do with persuading anyone to buy.