You're not wrong by any means, but it's also a question of scale and purchasing power. There's no doubt that r/parenting is filled with people worth talking to about, say, baby products. But then look at the advertising sales minisite for Babycenter.com[1]; a company like Johnson & Johnson isn't going to bother with r/parenting. Smaller companies might be interested, but small companies don't spend much money on ads. When they do, they mostly spend it on Adwords.
Of course the joke here is that J&J bought Babycenter. If I were a category-specific CPG company, I'd cut out the publisher middleman, too.
Ad networks & exchanges lead us to think there's a perfectly efficient & liquid market for ad space, but there's a real scaling problem for big advertisers that leads them to stick with big publishers. Quality control on r/parenting would be a concern.