I'm guessing here, but a lot of subreddits have a minimum karma requirement to make posts or comments (it's a decent anti-spam measure, but easily gamed by low effort bots reposting content every month or so, pretty high chance a lot of people won't have seen it before).

And as with other social media companies, the company behind it is fine with it; bot engagement and bot content is still content, people still watch it, upvote it, and ads and other things are still sold.