Many modern games take a long time to produce, 3-5 years is not uncommon for AAA titles. I also wouldn't say they haven't released a game 'for years', there has been at least 1 title a year, a ton of DLC for older games, porting games to linux, etc.

I agree that they are mostly a platform, but they have had at least one game come out every year (except 2002) since it started.

Recent ones: Portal 2 - 2011; Counter-Strike: Global Offensive - 2012; Dota 2 (to be released) - 2013