>>772279
I guess this is a consequence of a manga being owned more by than author, rather than a company as with western comics/cartoons. Take any popular comic book hero from Marvel/DC, no matter who came up with the first run, of said character, the character belongs more to the company, than to the author(and it's also not just an author, but rather a team, since one does the inking, one does the coloring, one does the story and so on, whereas a mangaka generally does everything himself), and at any point the original author can be replaced with any other author, that can do almost anything he wants with said character, even if he didn't contribute to the original run. As such, since all the characters belong to a company, cross-overs were a natural consequence, until it became one large shared universe.
Honestly, I don't know how much a mangaka owns his ip, and how much the publishing company, like Shounen Jump, does. A quick google search, brought me to https://anime.stackexchange.com/questions/38451/who-generally-owns-the-rights-for-a-manga-or-anime where the tl;dr is that the copyright is shared, so I guess the original author can't be replaced, unless he wants to, or sells his part of the ip. As such, shared universes or cameos, or cross-overs would most likely happen if all the "universes" were created by the same author, or if two authors are good friends, and decide to make 2-3 issues for a cross-over event, to boost sales or to depart from the norm a bit, which I guess can be fun for a mangaka once in a blue moon to try and co-create an issue with another mangaka, who he likes as a friend.