The Avengers never invited Spider-Man to join because Spidey's an emo cry baby.
The Avengers: Here comes Spider-Man! Ignore him, pretend to be playing Pokemon.
Spider-Man: Hi guys! Whatcha doing?
Hawkeye: Pika! Pika!
Hawkeye: Avengers, here comes Spidey, I'll stop him!
*throws box of Uncle Ben's Converted Rice at Spidey*
Spider-Man: Hey, low blow!
The Avengers: Leave, Spidey, we're battling Loki!
Spider-Man: I fought him in Amazing Spider-Man #236.
Hawkeye: You read your own comics?
The Avengers: This is Avengers HQs. Been here before?
Spider-Man: Once, at Halloween. You gave me candy.
Hawkeye: OK, don't cry, Emo baby.
Hawkeye: Sorry, Avengers aren't looking for new superheroes.
Spider-Man: You're a superhero? Were you bitten by a radio-active arrow?
Hawkeye: Talk to Nick Fury about joining.
Spider-Man: How do I reach him?
Hawkeye: Can't hear you over Hulk!
Hulk: Oh, yeah...ARGH!
Spider-Man: One day Avengers and Spidey will work together!
Iron Man: Yeah, on Hollywood Blvd harassing tourists.
Hawkeye: Aw snap!
Loki: As your brother, I'm disappointed in you. Spider-Man told me you wouldn't let him join The Avengers.
Thor: That little snitch!
Avengers: Hey Spidey, check out our new song: "Avengers, Avengers, Does whatever an Avenger does."
Spider-Man: I'll see you in court!
[Recycled Twitter Posts via Twitter and Witstream]