The reason we haven’t seen any Spiderman Avengers crossover is because Marvel gave the rights of Spiderman to Sony while giving the rights to make Avengers to Fox. Which are two direct competing companies! On top of that they have this possessive mindset of their properties and the studios dont usually like to share with each other. The companies are all about making money as their first second and third goal. But ironically they would both be making tons of money if people heard they were doing a crossover and fanboys would come flocking to the theaters to watch. The main problem is bot Sony and Fox cant even agree on how much percentage they should get if they put the cameo of their charterer in the others movie. So what they do is decide not to play ball at all. The easy way to solve this is to make a double crossover make spidy go to avengers for a cameo and make a few avenges come to a spidy film! boom done! everyone wins!