For the short sentences and paragraphs thing, I think it's about readability more than anything. People tend to scan content when they read online, not go through word by word, and it's probably easier to do that with short sentences and paragraphs.
It's also likely in part down to them optimising for readability tests. Stuff like Yoast grades your content based on the results of a Flesch-Kincaid readability test, which marks it as easier to read if there are less words and syllables per sentence, with the recommended score being roughly equivalent to a grade 8 reading level. Hence these SEO writers are basically writing for 13-14 year olds.
I think it's called direct response copywriting and mimics a more conversational style.
I know, you wanted to make a negative example and yes, this writing style gets old pretty fast. However, for a moment it works surprisingly well. And in a way the value you provide with that style is that the point you want to get across sticks better somehow.
Want to know?
It's simple.
They fall fowl of Goodhart's law!
But what's that? I'll tell you!
Goodhart's law states 'When a measure becomes a target, it ceases to be a good measure.'
That's a lot of words, and it's really complicated, so let's simplify!
Stay with me -- I'll teach you. We can learn together.
You're still reading this, but you're not learning anything.
And yet I'm being measured by you reading. To. The. End.
So I do anything to make you do that.
Anything.
It doesn't matter if I'm not adding value for you any more.
You've probably figured it out already and are skipping to the end. Thanks for getting this far.
Basically, if you aim to game metrics, you can do it. Easy.
But why were you aiming for organic searches in the first place?
You wanted your visitors to like you. To trust you.
And to meet that goal the 'writers who weren't SEO specialists' were actually your friend.
They were (if they were good) _reader_ specialists. They created what people wanted to read. Not what would make your KPIs go up.
And ironically, now your KPIs look great. But your content is bad, and not useful.
Don't aim for your measurement. Aim for your goals, and use your measurements to see how close you got.