Spider-Man has an actually interesting supporting cast/rogues gallery and has several aspects of his own universe that work wonderfully and work better when it's standalone.

The reason he's "excluded" is he doesn't need the Avengers/FF/X-Men and everyone in Marvel knows this.

I understand for new readers that Shared Universe Recognition and Synergy is some kind of bizarre status symbol but for most of Marvel's publication history the shared universe was just a means of getting the most out of every character Marvel had rights and license to. The Avengers from the 60s to the early 2000s was a dumping ground for rejects/D-listers and others who couldn't carry their own title.

The reason Spider-Man's world rarely crossed over with other characters is that he existed independently of them.