There is a reason I said that Sabrina is not officially the leader, mostly because she would deny being the leader, but regardless of Sabrina's self delusions other groups will notice that Sabrina is unofficially leading the group and react accordingly.
Honestly Sabrina seems to be the best fit for a leader anyway. Mami is too insecure and dependent, Homura is awful in the social department, Sayaka would be too quick to judge megucas outside of the group, Oriko and Kirika aren't able to lead the group, because Homura and Sayaka wouldn't follow them, etc.
I think there's two or a few reasons why we are not
the leader, despite how often we drive the group to pursue our own interests...
Well, first would be that, even though we do lead our group sometimes, that does
not make us a leader, not of a healthy group at the very least; it pertains a healthy group to allow any of its members to assume temporary leadership position whenever the subject they are dealing with is best dealt with by said member. And when we try to help magical girls, try to take care of or forgive our opponents, go out and break fights, we never
force everyone else to follow our lead; we ask, and many times our interests are simply aligned with that of the rest; when they are not, we
tend to not push the issue.
Say, we pushed Homura to spare Oriko because Oriko's and Kirika's lives were on the line, but at the same time we ofte
don't push her when her insecurities come into play, even if we might end up acting somewhat less than optimal because of it.
Beyond that, I'd say, and I think somebody already brough it up, we would
suck as a leader. Social is not our thing. Mami, however, is pretty good at it (she
can and often puts on a strong face when needed, despite her insecurities), and really, she's such a paragon of niceness and justice we know she won't lead this group astray, so... why wouldn't Mami be the leader?
Why
we shouldn't be the leader is, at least partly, because of our (Sabrina's, not saying literally every single person in the thread has) control issues. We have shown more than once our emotions, and almost our actions, go out of control when confronted. That time Hitomi tried to slap us being a prime example.