I get where you're coming from. Logically, it makes no sense to do that. But to keep a story alive. These type of restrictions have to be done. Because, if you really want to go with the logic. Than Spider-Man can call 9 people at a whim and own his villains. Just the top off of my head, Parker could easily contact: The Fantastic Four, Daredevil, She-Hulk, and a ton of Avengers that are mostly free (Wonder Man, Iron Fist, Squirl Girl, Vision etc.). Second, it's not done because, what happened with Spider-Man 3. Too many people, not enough time.
Now imagine lots of villains, lots of heroes. No time for the characters bits. That's why Avengers does what it does. It's for that portion. While the character bits are saved for their solo movies. Just assume when Cap 2 is going on. Everyone is doing something. Infact, when Iron Man 2 was happening. Incredible Hulk was happening at the same time. Some of the movies don't even follow chronological releases. It's just like how Batman's death could of been dealt with faster, if they called Superman or Wonder Woman for a favor. That spoils so many stories. They could write in scenes where the character does call people. But everyone turns them down. But that than just wastes the movie budget. With Iron Man, it could be a funny bit with JARVIS. Where he says: "I've already called everyone. No one got back."
