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.
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.