> What's more, ads served by the API to a client that isn't controlled by Reddit are less valuable - Reddit would have less information about how much engagement they had with the ad.

That's the key: Reddit and Twitter and Facebook and the like are not selling ads, they're selling you, the user. The users are the product. They're collecting every bit of data about you, building personal profiles. 3rd party apps would never be that intrusive. Delivering eyeballs on ads is not enough.