Because AI isn't so hot on the "I" yet, and if you ask it to generate this kind of document it might just make stuff up. And there is too much content on the internet to delve deep on whatever you come across to understand the soundness of it. Obviously you need to do it at some point with some things, but few people do it all the time with everything.

Pretty much everyone has heuristics for content that feels like low quality garbage, and currently seeing the hallmarks of AI seems like a mostly reasonable one. Other heuristics are content filled with marketing speak, tons of typos, whatever.