The Avengers used to be a pretty select club. Oh there were dozens of Avengers by the time I picked up my first issue, but back in the 80s, there was a rule than an Avengers roster could only count 6 members. Even when they doubled the active Avengers by instituting a West Coast team (which I loved), each team could only have 6 members (Hawkeye did bend the rules for a while and had Hank Pym consulting as a 7th member). Back then, if a member left, another would have to be recruited, and that was exciting. Perhaps I knew I couldn't be one of the 6, so I never bothered with filling out the card.
But today, it's quite the opposite. The Avengers have a massive roster, with anyone and everyone being an Avenger, someone Steve Rogers and/or Tony Stark can call on in an emergency, and there's always an emergency. Even if that weren't true, the Avengers brand is so popular that multiple teams, some not even affiliated with the official ones, carry the Avengers name. There are secret Avengers, and new Avengers, and dark Avengers, and mighty Avengers, and young Avengers, and assembled Avengers, and an Avengers academy, and an Avengers arena, and pet Avengers, and 1959 Avengers, and uncanny Avengers, and undercover Avengers, and all-female Avengers, and a whole Avengers Universe! Avengers who should never have been Avengers, according to hallowed and unspoken rules, Avengers like Spider-Man and Wolverine, are, yes, Avengers!
So now that it's no real feat, now that a number of morts are part of the Avengers Initiative, morts who might even make the Great Lakes Avengers look silly, there just doesn't seem to be a point to filling out a membership card. If they need me, I'm sure they'll draft me, no card necessary. I can even go off and join another team; it doesn't matter.
I'll just sit here and remember the good old days when becoming an Avenger was difficult, and I probably couldn't have done it.