They could start by getting rid of the need to root phones, to have full access to them: entering developer mode could give you access to something like sudo.
Security. Many people want to run critical applications on their handhelds, and the policy is to err on the side of safety.
(In "better safe than sorry", that is a different kind of sorry - I spoke only yesterday with a user that got locked out of the device completely because of a small accident.)
(Personally, I do not use said «critical applications» because I do not trust container, application and providers. So...)
All the more reason to find ways to break those restrictions.
Surely running a digital ID app on something like e/os is possible? If not there should be a lawsuit.
There's no technical reason why Android can't just be another Linux distribution. There's no technical reason why you could not boot into an OS from a usb key on your phone like a normal computer. There's no technical reason why the play services have more rights than the actual owner of the phone. It's all about who controls your device.
Android follows a "look, don't touch" model and will continue to be this way as long as possible on Google's side.
I thought that Android is mostly Java (that happens to run on a Linux kernel but might be anything as far as the specs are concerned).
Which means most Android drivers are actually written in a mix of C++, Java and Rust, with a standard ABI, HAL.