Much as I hate to say it, I kinda have to blame Wally West. Before his late 90s (you should excuse the expression) run in both Titans and JLA, membership in super-teams was an exclusive affair. If you wanted to join one, you had to quit the other, and generally the membership was held to a manageable level (with the exception of the Legion of Super-Heroes, whose entire M.O. is ‘Giant Super-Team’ and is thus exempt.) These days, nearly every hero in the Marvel Universe is an Avenger, even those who have for decades made a point of operating entirely solo. Indeed, given that there are five or more monthly titles bearing an Avengers logo, the team has easily surpassed the LSH in terms of heroes-per-month (which one might think negates the stupid arguments that the Legion is too complicated, but that’s another train of thought entirely.)
The MS-QOTD (pronounced, as always, “misquoted”) liked it better when characters weren’t allowed to hold simultaneous membership in multiple teams, asking: Does it bother you that everyone is an Avenger, including most of the Fantastic Four and a good portion of the X-Men?