Someone on Twitter made a similar comment and asked the reddit ads team if they would ever consider implementing any type of bidding system (I don't think they've responded).
Yeah, I asked personally and they told me the same thing. I also subscribe to the subreddits I would advertise on, and see any ads related to the category maybe 1/500th of the time (I don't use adblock). I've never seen it on the subreddit itself, which has over 20,000 subscribers.