Man, it sucks that I know you're not reading these because seriously, you are so far from the truth. All those stand alone heroes you talk about, had supporting cast.
Remember the end credit scene from Iron Man? "You are part of a much larger world, you just don't know it yet."
Well guess what, so is Peter AND he does know. He was recruited into a fight between the Avengers. OFCOURSE they are going to be part of his movies, because they are a part of his world. Tony was established as the one that found him, and Tony takes care of his own. Ofcourse he was part of the first movie. Fury (even though not Fury but Fury by proxy) knows of him, OFCOURSE he would recruit this young talent for a mission. Peter now needs help.... OFCOURSE he's going to ask an Avenger.
Spider-Man was always the focus of his own movies. Stark, Fury, Strange... in comparison to him they hardly had screentime. Your logic isn't logical, it's emotional.