Because code isn't prose. It has a much more structured, "correct" form than prose. After adhering to a high level structure of a program and applying linter rules, there's really not that much variance between code (In terms of how it reads, not correctness or performance). LLM code without guard rails can get really bad, but with some guard rails it's generally not worse than the average code I'd read before LLMs. I'm used to code being generally bad.

Books or blogs on the other hand are different. Every person writes differently, and the writing style is part of the experience. The same scene or concept can be described in an infinite number of ways. But LLM prose ends up very samey. You get the same sentence structure, the same key words, the same metaphors, the same similes, the same rough distribution of word usage. It's very average, and it's boring. And it frequently reads like a corporate blog.