I halfway agree with you - they probably wouldn't try to write something like Halo, because they wouldn't know where to start. Simple, fun games still exist though. Just look at all the Flash ones.
(Numerical comparison: Half Life life-time sales: 9.3 million as of the end of 2008[1]. Bejeweled life-time sales: 50 million+ as of Feb this year[2]. And how many play FarmVille (though it's not overly simple)?)
[1]: http://www.shacknews.com/onearticle.x/56193 [2]: http://www.gamasutra.com/view/news/27183/Bejeweled_Sales_Hit...