Maybe companies are afraid of losing user base by NOT releasing sequels every year? Plus using the same engine with tweaks you save money.
Not that that would make too much sense at first but hear me out - Call of Duty 2 and 3 did not sell as well as Modern Warfare and Call of Duty WAW. But how much would have Call of Duty WAW sold if it was released 2+ years after Modern Warfare? People might have "forgotten" about how good MW was if there were to be a large gap with no hype. Keep in mind Modern Warfare was based on ... Modern Warfare, and the old games were based on World War settings that were getting "old", yet WAW still sold like crazy!
Granted hype in the form of commercials and what not could have kept sales going, but maybe It's cheaper to pump out another game in a year using the same engine and what not. I think it would be anyways.
Back at hand, we see Left 4 Dead 2 using the same source engine with some tweaks so this is pretty much instant money for Valve.
I'll take a new Left 4 Dead any year over Madden 20xx.
It's just that simple.
