No revenue split would probably the biggest reason. Also Unreal could still pull a Unity since it is not FOSS (unlikely, but still).
Mind you, Godot people are working on optimizing engine performance (see my other comment) because they kind of rushed Godot 4 release.
> even a more mature C++ renderer framework like bgfx, or even Ogre3D?
IMO this is actually the way to go & it is probably the way I'd do it if I had the time to build editor-like tools by myself. But these days content is the hard & time-consuming part of building games. Which means you want non-coder gamedev to be able to do as much as they can, which implies a powerful editor. Godot & Unity have the advantage of pushing the game development to the editor instead of the code.