The reason for this organization is that many of the key components of the VFX platform are open source, but not open governance. They live inside Dreamworks, Disney, etc. but without any sort of public collaboration model. You can't contribute to the Alembic file format unless you work at Imageworks, you can't contribute to EXR unless you're at ILM, and you can't contribute to OpenVDB unless you're at Dreamworks.