Why would developers do that?
The developer's incentive is to control the experience for a mix of the users' ends and the developer's ends. Functionality being what users want and monetization being what developers want. Devs don't expose APIs for the same reason why hackers want them - it commodifies the service.
An AI-first app only makes sense if the developer controls the AI and is developing the app to sell AI subscriptions. An independent AI company has no incentive to support the dev's monetization and every incentive to subvert it in favor of their own.
(EDIT: This is also why AI agents will "use" mice and keyboards. The agent provider needs the app or service to think they're interacting with the actual human user instead of a bot, or else they'll get blocked.)