1 - Sony does NOT own Spider-Man in any intelectual way, they only own the rights to making movies about him.
2 - Disney chose, back in 2014, Sony to make Spider-Man games for an undisclosed amount of time (or a set quantity of games, maybe).
3 - This contract will probably expire sometime in the future.
4 - It doesn’t make any sense for Disney to have Spider-Man be exclusive to one platform, just like it didn’t make any sense for the MLB to have The Show exclusive to one platform.
5 - When the old MLB contract expired, a new contact was signed that forced Sony to make The Show multiplatform.
6 - Microsoft is not dumb and is doing everything they can (for example, having The Show on Game Pass) to make MLB The Show successful on Xbox. They are doing this so that Disney realizes how much money they are leaving on the table for no reason and forces Sony to make future Spider-Man games multiplatform when they write a new contract once the current one expires.