Not everyone is willing to jump into low level 3D APIs, the traditional CAD/CAM vendors for example.

Many think that OpenGL with core context is already too low level.

Which the answer is in both cases to have some kind of middleware.

I always considered a mistake that OpenInventor was never part of the standardization deal, because everyone ends up writing their own version of scene graph APIs.

Farenheit, Inventor, VRML, every so often one comes out with "the scene graph to rule them all". Turns out writing a high-performance renderer (and you do want that) is quite challenging. There are so many different rendering techniques and the scene graph is really not the hard part...

I do agree, the thing is 90% of the API users just want to render a mesh of some sort and not write a high performance render engine able to sustain 90 FPS under all conditions.