Something that works in a niche medium like comic books doesn't automatically mean success in a medium like film, which tends to hit a wider range of demographics.
Spider-Man has transcended his comic book roots to become a pop culture icon. Kids love Spider-Man without having to ever pick up a comic book. I believe he is the most profitable superhero of all time. Studios are more inclined to tap into that sort of mass-appeal with characters like this, than they are to cater to a demographic as limited as comic book fans. It's not comparable to a character like Wolverine.
I have the first issue that Carnage ever showed up. I am more than aware that Spider-Man, in the comics, has taken on violent serial killers. I am aware that he has been bloodied and beaten. That works fine when you're catering specifically to people who read comic books.
I am also aware that Spider-Man sells huge with children, and I do not see any scenario where Sony alienates that entire demographic just for the sake of throwing him into Avi Arad's stupid pet project. It stands to reason that they'd want to market Spider-Man's inclusion in a Venom film. How would that go?
"Spider-Man is in this movie. I'm sure your kids are thrilled. But don't take them, parents! This is super violent and gory, 'cause Deadpool and Logan!"
Actually, I take that back. Avi Arad is stupid enough to try that. Nevermind.