Advertisers buy the accounts, and use them to shill products. You know those posts where some product is being highlighted or is even just visible, someone comes along and asks what it is, and then someone links it? It's so they can do that. Maybe even have a few sock puppets to talk about how much they like it. If these were all fresh accounts, the scheme is easy to see through. If you look through their post history and instead find a 5 year old account, with hundreds of comments (good comments, upvoted comments), you or anyone else is less likely to think it's an advertisement. It just looks like a normal user.

And reddit doesn't care, because it's more content for the real users to browse, and to show off for an investor.