Technically they all (since CoD2) use IW Engine, but different versions of the engine. CoD:MW (the next IW-developed game after CoD4) did support custom maps, but the esports community never played it. As of MW2 (and possibly/probably the Treyarch CoD titles earlier than MW2), dedicated servers are no longer used, meaning no more custom maps. I'm not sure if that will still be the case when MW3 is released later this year, I don't have any marketing briefs to refer to yet, and if I have been told already then I've forgotten. (And, if I have, chances are I was also told not to repeat that information, so probably a good thing I've forgotten.)