It used to be super common that when you spotted a bot post and clicked through to the user's history, you'd see very average, human-looking activity from years ago, followed by a long gap of inactivity, and then a flurry of obvious bot comments.

It's very obvious that these accounts were abandoned and then either bought from their original owners, or more likely bought from someone who compromised them, because of their history and karma.

And I would bet money that Reddit is well aware of this phenomenon, because not long after it became so common as to be impossible to ignore, they papered over it by allowing users to hide their history from public view. (AFAIK subreddit moderators can still see it, but typical users now have much less ability to see whether they're interacting with actual humans.)