When I search via Quickmatch, I get the "Searching for Games" message followed by the "No Games Found" message. There must be games available. Why is this happening?!

If the "No Games Found" message is IMMEDIATELY displayed, then there genuinely aren't any games available in that mode. This might happen if you search a non-campaign mode at an off-peak time, for example.
If the "Searching for Games" message hangs for about 5-10 seconds, then returns the "No Games Found", then every game we tried to connect you to was full before you could join. Since our lobbies are 4 player max lobbies, they will tend to fill up quickly while people are searching. We keep searching for games for 5-10 seconds, and if they are all full in that time period, we return you to the lobby so that you aren't stuck in the search for too long. In this case, repeating your search will eventually get you into a game.
You could have issues with your NAT settings and/or your UPnP settings. Please review our Support resource page for some directions on what to do about this!