Think of it this way: if I wanted to commit pure academic fraud maliciously, I wouldn't make up a fake reference. Instead, I'd find an existing related paper and merely misrepresent it to support my own claims. That way, the deception is much harder to discover and I'd have plausible deniability -- "oh I just misunderstood what they were saying."

I think most academic fraud happens in the figures, not the citations. Researchers are more likely to to be successful at making up data points than making up references because it's impossible to know without the data files.