To some extent, Reddit does get a slice - in the form of user engagement. User engagement is how they generate ad impressions, even if it's not from the users on the third party apps.
They COULD have simply put ads into the API, or made it a requirement. They didn't.
Their entire goal is to maximize "value" before their IPO. Control and number inflation. They don't care about the long term. Spez wants to cash out, and he doesn't care what it costs the company.