Curation is expensive, and difficult/impossible to automate, but there's value in it, in a world where anyone can post anything anywhere. We all felt better using Amazon ten years ago, when we could trust reviews, and trust products, but now it's flooded with brand names like "UZOOG" or whatever. People generally liked Facebook more before their grandparents and weird uncle got on it.
The reason it works on reddit is because there is not money to be directly made by tricking the volunteer curators. If those reddit mods were controlling who could sell something, there would be a lot more effort going into corrupting them.
I still think you're right, Reddit has a much higher signal to noise ratio than almost anywhere else on the internet. But there's absolutely money to made by paying off mods of larger subreddits to curate content in such a way that makes their brands look good.
