This by the way was the entire reason Marvel invented crossovers. Lee wanted to get as many titles up and running as he could. The minute Spider-Man launched and proved himself, the appearances by other heroes became more sporadic and infrequent. Steve Ditko wasn't in favor of the Fantastic Four appearing constantly that Lee insisted on and repeatedly sought to downplay them.
The Avengers were for the longest time a dumping ground for B-C-D list heroes or villains who were so lame that people thought "hehn...might as well make them heroes". The shared universe in the classical marvel era was regarded by editors/writers/readers as basically a gentlemen's agreement. Everyone knew that the best characters worked best and flourished best as solos.