My unscientific wild ass guess would be that because of how LLMs are built to be pleasing, people wind up liking them more and thus lowering their guard with them and therefore judging them less harshly

For a concrete example of what I'm talking about

Imagine if you are really into older movies, like 60s and 70s movies

You start talking to two chat windows about your love for movies

One chat partner shares your love for old movies and is very enthusiastic and wants to talk all about them. In reality, this chat partner is the LLM

The other is lukewarm and maybe tries to steer you away from that conversation because they don't know much about older movies. Maybe they still love movies but they want to talk about more recent movies. In reality, this one is the human

But which one do you think is the human?

If you are self aware that your love for old movies is not really universal, and you are aware that LLMs have a tendency to match enthusiasm, you can probably guess which one is which

If you are less self aware, you are probably just going to guess that the conversation you enjoyed more is the one with the human