Dunno about co-op, but they feel that searchable custom games would make people stop playing the other modes.
But with Halo's gigantic community, it makes no sense. Far smaller games do well with Ranked and Unranked playlists, plus successful games like Gears and CoD4. If they don't want to detract from their stupid searches with the same 3 maps coming up and only 5 gametypes represented, then just make it a server list instead of matchmaking.
It's especially stupid since it gimps the Forge. Lack of custom search in Halo 2 wasn't as bad because I was younger and it was easier to get friends on, there were less shit-hot XBL games, and the game wasn't so built on the aspect!
>smh<
