That excuse doesn't work as well when you consider doom 3 and halo for the xbox had coop. Can you tell me why the halo port to the pc had no coop? Figure that out and you will know why pc games have no coop anymore, it sure as hell ain't any technical reason.
There was a technical reason - the coop scripting in Halo was set up to take place on a single box, with no packets being sent to a second machine. This was believable, since Halo didn't support coop over system link, much less Live, even though it would have massively benefited from the feature.
Still sucked, though. It was totally the make or break feature for me, and not having it = no sale for me.