Speaking with two people familiar with the situation, The Information reports that development on a new VR/AR OS had been in the works for several years and involved hundreds of employees.
Meta currently uses a modified version of Google's Android OS to operate its Oculus line of VR headsets. Meta reportedly wanted to create its own OS, internally dubbed "XROS," in order to avoid the technological and logistical hurdles that come with using a third-party OS. That would include having to update your own modified OS any time Google updated Android to address security issues or make other changes. Changes to a modified Android OS could also cause technical issues to come to light, since the OS was originally designed for smartphones but adopted for other hardware, like VR headsets.
https://www.ign.com/articles/meta-vr-os