> I've wondered what in the training process causes the model to develop this peculiar style of writing

I think this might just be inherent to the currently deployed LLM architecture. If the training process works at all, any model will come out of it with some (default) voice that it'll apply everywhere.

Human voices are not like that, both because one copywriter does not write a significant fraction of all texts in the world and because unlike LLMs they remember all of their own prior writing, which gives them an opportunity to get tired of their old tropes.