Earlier quoted context omitted.
--- Also, why pay to advertise when you can just make a shill account for free? --- Because it does not guarantee visibility. "Why pay for advertising on Facebook when you can have a Facebook page?"
Because the advertising model is very weak on Reddit, it's just a sponsored post with no differentiation most people skip past. No video or image to catch interest. Plus, the users are self segregated, what a dream. Imagine you wanted to sell drop shipped backpacks. All you'd need to do is create and prime, or buy, a dozen or so accounts, and start recommending your own brand subtly in every recommendation thread in…
That's what advertisers would like their customers to believe.
The truth is closer to: advertisers are lazy, Reddit users aren't complete idiots, subreddits are moderated, and spammers are (rightfully) despised. Effective shilling takes more effort in actively moderated environment so I wouldn't be surprise if sponsored posts had a better ROI than the much more dishonest plays.