Because when you're matchmaking, Everyone has their own idea of how to deal with the bosses unless everyone is good at communicating and super open to ideas. When you form your own fire team, you got a selected group who do the same things together in depiction in which what works and what doesn't. It's more effective to have a selected few who you work closely with than matchmaking with complete strangers. As for filling that last spot, then I don't know what to tell you. VOG and weekly strikes are suppose to be more "intimate" with your selected group. You're suppose to do things together as a group to be an effective fireteam. Does that make any sense?