At least one part of it is that stories sell.
Andrew and Jenny, like many people in digital marketing, ran some
sites on the side to get a little extra cash. They were sitting in the
pub one day and chatting about how they were each doing.
"I've just made the final payments on my car," beamed Andrew, "and
it's all down to my side project".
"What?" sputtered Jenny. "I haven't made anything on mine in weeks.
How'd you make it work?"
"Simple. I tell people a story about the product. A nice big chunk of
prose, sometimes even with dialogue."
Jenny frowned. "Don't people just skim most of what they read online?"
"Oh yeah, sure. But what they don't tell you is that once you can get
people reading about something, you can hook them with a well-written
story about the product."
"And then BAM," he slapped the bar, "you've got them. People love
stories, they put themselves in the position of the characters, it
makes the whole thing seem much more relateable. And not only that,
you can talk about the benefits of the product and answer any
objections while you're doing it."
"And then people will just buy?"
"You'd be surprised. Check out this book by dhmholley, he's got all
the secrets you won't learn in a marketing class. You can buy it on
ux.stackexchange.com and it'll tell you everything you need to know
about writing stories."
Of course, stories aren't the only type of long form content, but the others typically have the same attributes - extolling the virtues of the product and answering questions and objections as they go, and tapping into the experiential appraisal done when the brain is taking in information. You can even drop in other tried-and-tested neuromarketing methods in the same space - in particular testimonials rely on anecdotes overriding rational processing and appeals to authority.
Notice how the top of those pages are frontloaded with the bare bones and some "calls to action" (CTAs), and then the sales pitch comes for those who scroll down? It's because once you've started scrolling you've demonstrated interest, which is the hook for the sales pitch. Obviously it's much harder to sell with prose to people who aren't yet engaged. Conversational writing is something else you'll often see, as it comes across as more trustworthy (or at least, more relate-able) to certain people - this lends itself well to longer pieces where the author can develop a rapport with the user.
It's also about selling to your audience. A person who is going to buy a gimmicky product probably already responds better to certain advertising techniques, which wouldn't necessarily be the same ones you might use on other users. Very few people are rational agents, so sales techniques like this work surprisingly well on a lot of people - most decisions can be manipulated by appealing to emotion, forcing the user to justify it to themselves later. In addition, if you're targeting a niche like this, you don't really need a high success rate as your costs are typically very low.
Lastly there are the SEO benefits of lots of content, which gives them a better opportunity to drop keywords and additional links to more content. This isn't to be underestimated, since there's a lot of potential traffic on these keywords.