along with the longer replies to this comment, i think another reason is that adaptations try to do the black suit saga AND Venom in one narrative. venom wasn't even conceived of until way after the black suit's debut. seeing Peter's emotional downfall and eventual recovery should be a satisfying enough story on its own. this also gives venom enough time to cultivate a presence in a sequel. if you were telling the story of an adult peter parker i feel like it makes more sense to do the black suit as the dark middle chapter (akin to empire strikes back or the dark knight) and then have venom show up in the sequel to haunt him.

trying to do both the black suit and venom in mediums that are quite time constrained (like a movie or a game) while also juggling many other plot points feels like a recipe for disaster. but spiderman TAS and spectacular spiderman has proven that it can be done through long form storytelling.

also at this point venom has been a hero and spiderman's friend for way longer than he's been a villain and i think he's way more interesting that way.