I've seen ads on Twitter that render in a more sophisticated way than tweets. For example, they include buttons. Third party apps could implement this kind of

I've seen ads on Twitter that render in a more sophisticated way than tweets. For example, they include buttons. Third party apps could implement this kind of thing, for sure. However, on an ongoing basis, Twitter would need to ensure that all third party apps had the latest ad presentation logic, and that they worked correctly, and none of the third party apps used sneaky tricks to hide the ads. After all, there's no benefit to the developers of the third-party app in hosting ads that benefit Twitter. It would quite costly and fiddly for Twitter to administrate this, and IMO, the benefits of a third-party app ecosystem don't really stack up from Twitter's PoV.