http://insomnia.ac/commentary/state_of_ ... ing_media/
A great piece, BUT on the subject of co-op, you may be mistaken -- it really depends on whether or not you meant "networked co-op" or "same-computer/console co-op".
Adding networking to an existing/in-development game is one of the most non-trivial changes you can make, and if there hasn't been any planning in anticipation of this it can be virtually impossible (without a total re-design of the code from the ground up).
Even _if_ networking has been planned from the start, it's still a huge draw on resources -- for smaller downloadable games (say, XBLA) the effort required to support networked multiplayer can be at least half of the total programming effort.
Of course, if you only meant non-networked co-op then your point is valid -- depending on how well the software was designed it could still be a huge problem, but in this case it's not a problem a priori, it's due to laziness and/or lack of foresight, and in the best case it should be a trivial change.