I've got a degree in marketing, and I can tell you that these are all important things for your marketing branch to do. But if you got money, they shouldn't be the only things you do.
Here is an ad from Warhammer 40,000... remember, this is not an ad for miniatures (which they also do a lot of) this is for a rules book. It takes a little art (reused from the book with a little movement), some voice-over, and bam pretty decent ad.
I see no reason why D&D doesn't send these out on YouTube and social channels, targeting the same (or similar) audience they send ads of Magic the Gathering to. I understand they've had enormous success so far, but this type of targeting can only improve sales. It's honestly leaving money on the table, and I can only assume that the WotC team hasn't bothered because they're getting double-digit growth anyway.