New Guitar Hero packages "yearly"? I think you missed a few, starting from World Tour:
Guitar Hero World Tour: October 26, 2008
Guitar Hero On Tour Decades: November 16, 2008
Guitar Hero Metallica: March 29, 2009
Guitar Hero On Tour Modern Hits: June 9, 2009
Guitar Hero Smash Hits: June 16, 2009
Guitar Hero 5: September 1, 2009
Band Hero: November 2009
Guitar Hero Van Halen: November/December 2009
That's 8 games in a little over one year.