Mainly because this is one of the problems of a shared Marvel Universe, particularly one in which all of your super heroes know one another and are all friends–why isn’t absolutely every story a humungous crossover with every character in it?

Even on smaller things: if Peter Parker is having trouble paying Aunt May’s rent, can’t he just get help from Tony Stark?

You want your heroes to be the heroes, so it’s not really story-fun to see a bunch of X-Men show up to solve a Spider-man problem–you really want to see Spidey get himself out of the jam. otherwise, what’s the point?

And conversely, you don’t want to see Spider-Man or other heroes solving all of the X-Men’s troubles, especially the ones that stem from anti-mutant prejudice, as that begins to seem like parochialism.