Are you sure?
It might be seldom, but The Dark Knight Rises teaser trailer came out in July 2011 (being attached to Harry Potter and The Deathly Hallows: Part 2). Which is a full year before the release of the film.
If I remember correctly, both teaser trailers for Batman Begins and The Dark Knight followed the same pattern. It's not completely uncommon for a summer blockbuster to debut a teaser a year before the release of the film.
Yes, it happens on occasion, but most movies don't get a teaser a year in advance - that's the same thing as saying it happens, but only seldom.
As I said before, I expect we will see a Man of Steel
teaser with TDKR
. But we'll probably only see the first teasers for most of next summer's films this fall.
We saw the first trailer for The Hobbit a year in advance.
Yes, and it was the only December 2012 film to get a teaser a year in advance. The others either haven't had a trailer at all yet or only got their first trailer in the last month or so.