The sepia-tinted color scheme and the way LLMs overuse rounded corner tiles to the absolute death is part of it. Really what it comes down to, is that these LLMs have a design vocabulary of like 15 traits. None of the websites they generate have every single trait, but they are composed of some subset of 10 such traits, so it's still overwhelmingly obvious the moment you open the page, even though each one is slightly different. Humans might use 1~5 of those traits, but they'll also be mixed in with a much wider set of traits with more individuality to them.