I do not understand why ISAAC came to be when there is ROS. The time and resources that have been placed into ISAAC could of been put towards ROS development like other tech leaders such as Amazon and Microsoft are doing. Instead we have ISAAC which is a closed and proprietary source that reinvents the numerous wheels provided by ROS and Gazebo.
While the UE4 license is proprietary, you do have access to the full source code, once you sign up. There is a 5% royalty if you commercialize your product based on the UE4 engine. Epic Games put in a phenomenal amount of support behind the engine, in terms of constant updates to the engine sources, tutorials, roadshows and evangelization.
Isaac SDK and Isaac Sim is built to help developers use the capabilities of NVIDIA’s graphics/computing/AI platforms (with RTX/PhysX/CUDA/CuDNN/TensorRT) for robotics application development / work flows (both cloud and edge). It is complementary to all that you have mentioned above (such as UE4, Unity, ROS, Gazebo etc.).
Regarding the question “Why do we have the Isaac simulator instead of adding Structured Domain Randomization and the missing features to Microsoft’s Airsim?”