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.
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.
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.