I realise that it's ripe for mis-use as clients could always just not display ads, and analytics would be hard as even if the client isn't actively malicious, it may fail to display an ad that it records as visible which is effectively ad-fraud.
Nevertheless I feel like there are unexplored options here, including SDKs rather than web APIs, select partnerships, and maybe more. I would imagine if it could be done it would work well for Twitter, Reddit, and potentially even Facebook and Instagram.
I guess it probably comes down to it being a hard problem with little perceived benefit over owning the customer interface, but the backlash to these things always feels significant (in my bubble at least), and I'd be surprised if these companies didn't feel it was an unqualified positive change.
I've undergone audits for "Sign in with Facebook" usage in the past on a small app (~50k FB auth'd users), and it was enough of a spot-check that they probably catch egregious mis-use with not a lot of effort.