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.

> creating Private APIs for connected device

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.