Because the DMA legally obligates them to share those APIs when they are necessary to implement a feature for a connected device. The goal of the regulation is to promote healthy competition for connected devices by outlawing self-preferencing by massive players. Reasonable people can disagree about the goals or the downstream effects of the DMA, but creating Private APIs for connected device features absolutely falls under the umbrella of self-preferencing.
In the same way, the EU could ask manufacturers of wireless headphones to open up and homologise their proprietary “APIs” with which they communicate with the other earpiece so you can mix&match single earpieces from different manufacturers.
The point of this regulation (DMA) is to enable more competition in important market segments. If this exact thing becames somehow very important, sure, it's possible, otherwise it's a bit contrived. What's the point?
Forcing standardization and interop is obviously good for interop, but it's bad for companies trying to innovate, because it ties their hands. The moment apple ships a v1 they have to ship an API, and then they have to support that API and can't change it. When it's private they can figure it out.
Apple already spends years in R&D before releasing anything. Many of their R&D devices never see market. Requiring them to share an API they've actually shipped to paying customers is not a significant additional hurdle. We know how to version APIs now. They can still make improvements to public APIs without hurting anyone.
Which is why DMA only applies to huge, dominant companies (the complete list: Alphabet, Amazon, Apple, ByteDance, Meta, and Microsoft) and there too it does not apply to all technologies, only for those where standardization is important to enable competition. It's much more important to have at least some competition than letting dominant companies monopolise entire markets through 'innovation' with private APIs.
