Why is Spider-Man left out of The Avengers? [duplicate]

The simple answer, believe it or not, is that Sony owns Spider-Man, and Disney owns The Avengers. Avi Arad, founder of Marvel Studios, has said a number of times that the only way he'd lend Spider-Man to Disney is if Spider-Man runs out of story lines. And that's not likely to happen. He's got Venom, he's got The Sinister Six, he's got Carnage... Too many stories to tell.

Could it happen if the money is right? Everyone has a price. But at the moment, Avi has no plans to make Spider-Man an Avenger.

Sony owns the rights to Spider-Man (and Venom), 20th Century Fox has Fantastic Four, Silver Surfer, Deadpool, the X-Men (including Wolverine) and Cable. Marvel Studios (now owned by Disney) owns the rights to all other Marvel Characters.

Something to keep in mind is this only applies to the film rights. The reason the rights are spread all over the place is that Marvel Comics tried to make films based on their characters throughout the late 70s and 80s. Most of them were low-budget and did poorly at the box-office (if they made it that far). After losing a bunch of money, other studios approached them about licensing those characters. Marvel said yes and then those companies proceeded to make tons of money by making the movies right (see X-Men, Spider-Man, etc). Once Marvel saw that, they ended up starting Marvel Studios and have since gone on to make some great films.

However, by then they couldn't get back the film rights that they already sold off. Unless Spider-man films and X-Men movies start losing money for Sony or Fox, it's highly unlikely that they will ever let those rights go back to Marvel.

The rights to The Avengers belong to Walt Disney Co, however, the Spider-Man movies are made by Sony Corp, and they have no intention of selling the rights back to Marvel/Disney.

But,

• How Spider-Man Was Almost In 'The Avengers' ... Sort Of

  ... Marvel Studios and Sony [had a] discussion about inserting Oscorp into The Avengers skyline. The Unified Marvel Manhattan Skyline almost existed.

  By the time the Oscorp building was fully designed, The Avengers digital Manhattan was already basically rendered and there was some up-conversion that needed to go down, so - for timing - it was scrapped.

  BUT - Sony and Disney were going to let this happen. THAT's the key thing here. Timing just didn't work out.

From an interview (April 2014) with 'The Amazing Spider-Man' producer Avi Arad:

But if we want to do that, the crossovers, it has to be a story that is absolutely centered on Spider-Man. We cannot be second banana to anything out there. Because this is the king. This is the one that influenced young people from birth.

I’m not preaching, but Spider-Man, Peter Parker, who is in all of us, is too important to go in and use it as a sidepiece for corporate purposes. The studio may disagree with me, some fans may disagree with me. I don’t care.

(since Sony owns the rights to 'Spider-Man' and not Avi Arad, it's really up to the studio whether a crossover will ever happen)