You can also ask this question: why do people sell micro niche sites and keywords when they can just bank off of them?
It's because:
1. "Earn $1000+/m passive" is just pure marketing bull crap and they probably couldn't get a site ranking and banking like that themselves which is why they're selling premade sites or "dreams."
Selling stuff to "make money" is a common way to make money easier, as it's a fail-proof way. You write, deliver, get pai. With a site of their own, however, you must write, pay for hosting, domain, links, etc, and then wait for the hopes of ranking, then wait 2 months to get paid.
2. The writing/keywords they sell are also shit. Those "high quality" writers have tried making a site but can't get it to rank or make visitors convert, so they are just selling their shit writing to customers. Same with keywords, some are never meant to be ranked with a low budget and the BSTs that state "you only need content" are pure bs, they'll never be able to rank the keywords they're selling with no links either.
But overall, it's different on a case by case basis, but the observations I made above are for the sellers in general, so please don't take offense the actual good writers out there.