(But I don't want to come across as disparaging *all* groups that have settled into that mode: it seems that plenty come by it naturally. In a group of people one may be most motivated/energetic when it comes to making the regular get-together happen. And that person's personality may also be well-suited to learning more rules and doing more "work" away from the table, so they gravitate toward GMing.
Not a bad thing, in isolation. it's when the assumption is that one type of leadership *must* be coupled with the other that I start to break out in hives.)