Reddit Removing the Ability to Opt-Out of Ad Personalization

I’m surprised reddit doesn’t allow advertisers to place ads on specific subreddits. My best guess why is that advertisers would need to be able to decide what subreddits to put their ad on, and that’s maybe not as easy as it sounds.

Let’s say my I sell mechanical keyboards. I should be able to find r/MechanicalLeyboards pretty easily but that alone doesn’t give me a huge audience. What about mechanical keyboard-adjacent subs? I could guess a few, like r/computers but I’m quickly getting out of my niche. I’d need to know my way around reddit really well, or have access to some pretty sophisticated subreddit search to find enough communities to advertise on. So why wouldn’t I just let reddit do that for me?