From what we’ve learned, the reasoning is that while Spider-Man: Homecoming was technically a Sony production, the character exists in the Marvel Cinematic Universe, while Venom does not.
So how exactly is Spider-Man going to become part of the same universe as Venom?
The new Venom movie finds itself in an odd place, because while the title character’s origin has always been linked directly to Spider-Man in the comics, that won’t be the case in the film.
However, recently it was implied that one of the reasons that Venom obtained a PG-13 rating was so that it would be easier to make a future clash between these two characters work for a broader audience.