How wasn’t this tested and shipped with all the code needed to make it work out of the box?
It was tested but the problem was that because people were so keen on getting the VR to work the Oculus OpenXR implementation wasn’t ready.
Only a workaround was in place at the time of VR release for Oculus and that’s why they initially said that the MSFS VR would only work with the WMR headsets while rest would come later. They did a mistake for trying to cater for eager people who don’t know how to work with instructions like “put Oculus software on beta channel” and “edit registry to make OpenXR look for Oculus OpenXR runtime”. Unfortunately it is what it is. An out of the box ecperience for Oculus will come in time when both OpenXR and Oculus makes their software work with each other without too much hassle.