You’re more right than you probably realize!

It's more likely that this is from the training data if they're being trained on reams of Internet stuff.

I always thought it could be because volume-wise, most English prose is probably marketing copy and actual clickbait; so when you train on the entire Internet, you get a troll adept at writing ads. Then people ask AdBot2000 to write a novel and are upset it reads like the next iPhone launch site.