Another explanation is that they are copying comics, in that they always had cross-overs. The difference is, that back then you would have 10-30 maybe even more individual stories for Spider-Man, then one crossover with Iron Man or Fantastic Four, then it's back to 10-30 individual stories with Spider-Man. The cross-over was more of a special event or just a "guest starring DareDevil", but it would almost always return to the individual stories. This also worked for the TV shows/cartoons, the 90s Spider-Man cartoon had 65 episodes, most of which were dedicated to Spider-Man, but with 1 episode for X-Men, 1 for Dare Devil, and a few more for Captain America as that was a special event, plus one where he gathered a gang of heroes to fight in a cosmic battle, but in the last episodes of the season it was still Spider-Man fighting his rogue gallery. This is how the cartoon ended, Spider-Man and other Spider-Men from alternative universes teaming up to defeat Carnage that possessed another Spider-Man, it was all about Spider-Man, not Iron Man or Captain America.
Movies, do not have this luxury, they can't just release 10 Spider-Man movies and then have the cross-over with Iron-Man, now everything either gets a solo movie and then goes into the collab, or starts in the collab and then maybe gets a solo movie(Black Widow). They are rushing into the collabs, because, unlike with TV shows, movies take too long to be made and after the collab, they never go back into the solo stories, but all subsequent stories will feature X from Y movie.