Advertising copy is likely overrepresented in the general corpus of texts from the Internet that most LLMs are trained on. Plus, it isn’t a genre where truth matters all that much. It’s intentionally vague and cliché-riddled even when humans are writing it. So it’s something that I’d expect LLMs to be pretty good at creating.

“Garbage in, garbage out” is just fine if garbage was your desired output to begin with.