Originally Posted by
Kaitou D. Kid
I think the problem with Spider-Man being an Avenger is that he is too anti-Establishment for the Avengers. He is a working-class guy who mocks authority. He has a strong sense of responsibility. He started out as a metaphor for youth independence. None of that meshes well with a narcissistic industrialist, a prince of Asgard, an elite scientist, or government agents.
However, you could argue none of this is true about MCU Spider-Man. As antithetical to Spider-Man as some fans might consider him, he could technically work as an Avenger.