co-op matchmaking

I have loved every second of the game so far. Loved the campaign, loved the multiplayer and I haven’t been able to get into any FPS multiplayer in a few years because I was so burned out on it. But then I decide to start going for the co-op achievements and discover that there is no matchmaking for campaign?? WTF? Why have online co-op and no way to find partners? That makes no sense at all. I don’t want to go through the pain of trying to find people that want to do the same things I want to do at the same time I want to do them. I just want to sit down when I feel like playing and jump on with somebody and play a level or two. To not have this as an option is terrible. Is there any explanation why this isn’t in the game? Co-op has always been a big part of Halo for me and now it’s pretty much impossible to use.