Is there a reason they can't just do these movies like Spiderman, Sony pays and Marvel make them & get to borrow characters? Symbiote suit in Infinity War would be great.
Because Marvel is honestly concerned that superhero fatigue may become more than just a joke, and want to avoid flooding the market with MCU films. They're happy doing 2-3 a year and that's it.
And while Spider-man is a big enough character that they're willing to forego doing one of their own films in one of the slots, Venom and the like aren't important enough for them to bother.