Yeah but the CGI in fantasy movies is attempting to look real. In animation, nothing on the screen is real and I’m not expected to believe otherwise.
I think the word "real" is probably obscuring whatever the underlying logic is here. It's not attempting to look
photorealistic, but it's trying to look "real" even more than most fantasy, since it's depicting real things. For example, we can use your example:
Peter Jackson wants me to believe that this hideous and obnoxious thing is real and is really interacting with humans (or hobbits or whatever):
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Gollum is not real, and not even based on a real thing, so there's no gap between reality and fantasy to bridge. He can't fail to look like the thing he is the only example of. Remy in
Ratatouille, on the other hand, is depicting a real thing. In that sense it seems to require more suspension of disbelief, not less, since you're being shown a thing that you know exists and doesn't actually look like that, versus being shown a completely invented thing.
Films that take place in the real world have to match the rules and nuances and subtleties of a reality we know extremely well, but the fantasy world is
teaching us those things. To whatever degree we see it as unrealistic is,
by definition, the degree to which we have smuggled reality and our expectations of it into the film.
I also feel compelled to note that Gollum is
supposed to be hideous. I get why someone might not want to experience that, but that's not really about the fantasy aspect of the film, since it has obvious non-fantasy analogues (like drug addiction). That uneasy coexistence of disgust and pity is difficult and unpleasant, to be sure, but it's totally inseparable from the story.
I wonder if the introduction of the word "interacting" is getting at the real fulcrum of all this, though. That kinda suggests it's not about suspension of disbelief for any fantasy
concepts, or even just CGI (since most films have some now), but really just about the relatively narrow case of animated beings interacting with non-animated ones.